Problems #26-50

All Examples & Problems (no solutions)

**Problem #26:** A 0.204 gram sample of a metal, M, reacts completely with sulfuric acid according to:

M + H_{2}SO_{4}---> MSO_{4}+ H_{2}

A volume of 213 mL of hydrogen is collected over water; the water level in the collecting vessel is the same as the outside level. Atmospheric pressure is 756.0 torr and the temperature is 25.0 °C. Calculate the molar mass of the metal.

The vapor pressure of water at various temperatures can be found in this table.

**Solution:**

1) Use Dalton's Law of Partial Pressures to determine the pressure of the dry H_{2}:

756.0 − 23.8 = 732.2 torr

2) Converting that to atm gives:

732.2 torr / 760.0 torr/atm = 0.9634 atm

3) Use the ideal gas law:

PV = n RT(0.9634 atm) (0.213 L) = (n) (0.08206 L atm / mol K) (298 K)

n = 0.0083915 mol H

_{2}

4) From the balanced equation, 1 mole of H_{2} is formed when 1 mol of metal M is reacted. Therefore, the moles of metal that reacted was:

0.0083915 mol M

5) The molar mass of M is:

0.204 g / 0.0083915 mol = 24.3 g/molThe metal was magnesium.

**Problem #27:** A common way to obtain a pure metal from its impure metal oxide is to react the oxide with carbon, expressed generically as:

2MO(s) + C(s) ---> 2M(s) + CO_{2}(g)

If 5.00 g of an unknown metal oxide (MO) reacted with excess carbon and formed 738 mL of CO_{2} at 200.0 °C and 0.978 atm, what is the identity of the metal?

**Solution:**

1) Using PV = nRT, solve for moles of CO_{2}:

(0.978 atm) (0.738 L) = (n) (0.08206 L atm / mol K) (473 K)n = 0.0186 mol

2) Determine moles of oxygen atoms in metal oxide:

For every one mole of CO_{2}produced, two moles of MO were consumed.1 is to 2 as 0.0186 is to x

x = 0.0372 mol

3) Determine molar mass of MO:

5.00 g / 0.0372 mol = 134.44 mol

4) Determine atomic weight of M:

x + 16.00 = 134.44x = 118.44 g/mol

M is tin.

**Problem #28:** A compound of P and F was analyzed as follows: heating 0.2324 g of the compound in a 378 cm^{3} flask turned all of it to gas, which had a pressure of 97.3 mmHg at 77 °C. Then, the gas was mixed with calcium chloride solution which turned all of the F to 0.2631 g of CaF_{2}. Determine the molecular formula of the compound.

**Solution:**

1) Molecular weight of the gas:

(97.3 / 760) (0.378) = (n) (0.08206) (350)n = 0.001684967 mol

0.2324 g / 0.001684967 mol = 138 g/mol

We will use this to move from the empirical formula to the molecular formula.

2) Mass of F:

(0.2631 g) (38.0 g / 78.074 g) = 0.128055 g(38.0 g / 78.074 g) is called a gravimetric factor. It is the decimal percent of F in CaF

_{2}.

3) Moles of F:

0.128055 g / 19.0 g/mol = 0.00674 mol

4) Moles of P:

0.2324 g − 0.128055 g = 0.104345 g0.104345 g / 31.0 g/mol = 0.003366 mol

5) Empirical formula:

P ---> 0.003366 / 0.003366 = 1

F ---> 0.00674 / 0.003366 = 2PF

_{2}

6) Molecular formula:

PF_{2}weighs 31 + 38 = 69138 / 69 = 2

PF

_{2}times 2 = P_{2}F_{4}

**Problem #29:** A metal chloride reacts with silver nitrate solution to give a precipitate of silver chloride according to following equation:

MCl_{2}+ 2AgNO_{3}---> M(NO_{3})_{2}+ 2AgCl

When a solution containing 0.4750 g of metal chloride is made to react with silver nitrate, 1.435 grams of silver chloride are formed. Identify the metal.

**Solution:**

1) Determine moles of AgCl produced:

1.435 g AgCl / 143.32 g/mol = 1.001256 x 10^{-2}mol AgCl

2) Determine moles of MCl_{2} consumed:

1.001256 x 10^{-2}mol AgCl x (1 mol MCl_{2}/ 2 mol AgCl) = 5.00628 x 10^{-3}mol MCl_{2}

3) Determine molar mass of MCl_{2}:

0.4750 g / 5.00628 x 10^{-3}mol = 94.88 g/mol

4) Determine the atomic weight and identity of M:

The atomic of M will be:94.88 − (2 x 35.453) = 23.97 g/mol

M is magnesium

**Problem #30:** An unidentified metal M reacts with an unidentified halogen X to form a compound MX_{2}. When heated the compound decomposes by the reaction:

2MX_{2}(s) ---> 2MX (s) + X_{2}(g)

When 1.12 g of MX_{2} is heated, 0.720 g of MX is obtained along with 56.0 mL of X_{2} gas (at STP).

a) What is the atomic mass and the identity of the halogen X?

b) What is the atomic mass and identity of the metal M?

**Solution:**

1) Calculate moles of X_{2} collected:

PV = nRT(1.00 atm) (0.0560 L) = (n) (0.08206 L atm / mol K) (273 K)

n = 0.00250 mol

2) Calculate mass of X_{2} collected:

1.12 g − 0.720 g = 0.40 g

3) Calculate molecular weight of X_{2} and determine identity of X

0.40 g/0.00250 moles = 160 g/molhalf of 160 is 80, the atomic weight of X

bromine

4) Determine moles of MX:

MX : X_{2}molar ratio is 2 : 1therefore, 0.0050 moles of MX

5) Calculate molar mass of MX:

0.720 g/0.0050 moles = 144 g/mole

6) Calculate atomic weight of M:

144 g/mol − 80 g/mol = 64 g/molecopper

**Problem #31: **A metal sulfate has the formula M_{2}SO_{4}. 10.99 g of the compound was dissolved in water to make 500.0 cm^{3} of solution. A 25.0 cm^{3} sample was removed and reacted with an excess of BaCl_{2}(aq) to produce a precipitate of BaSO_{4}, which when dried had a mass of 1.167 g.

a) Determine the number of moles of BaSO_{4}precipitated.

b) Determine the concentration of M_{2}SO_{4}

c) Identify M

**Solution:**

1) Moles of BaSO_{4} precipitated from 25 cm^{3}:

1.167 g / 233.4 g/mol = 0.005 mol

2) Moles of BaSO_{4} that would have precipitated from 500 cm^{3}:

0.005 mol is to 25 as x is to 500x = 0.100 mol of BaSO

_{4}

3) The molarity of the M_{2}SO_{4} is this:

0.100 mol / 0.500 L = 0.200 MThe 0.100 mole comes from the fact that there is a 1:1 molar ratio between M

_{2}SO_{4}and BaSO_{4}M

_{2}SO_{4}+ CaCl_{2}---> BaSO_{4}+ 2MCl

4) What is M?

0.1 mol of sulfate weighs 96.061 x 0.1 = 9.6061 g10.99 − 9.6061 = 1.3839 g of M

^{+}in solutionThere were 0.2 mol of M

^{+}in the 500 cm^{3}[from this: M_{2}SO_{4}(s) ---> 2M^{+}(aq) + SO_{4}^{2-}(aq)]1.3839 g / 0.2 mol = 6.9 g/mol

Li

_{2}SO_{4}

**Problem #32:** An element, X, forms two compounds with bromine: XBr_{2} and XBr_{4}. When 10.00 grams of the XBr_{2} is reacted with excess bromine, 14.35 g of XBr_{4} is formed. Identify X.

**Solution:**

14.35 − 10.00 = 4.35 g of Br_{2}reacted4.35 g / 159.808 g/mol = 0.02722 mol of Br

_{2}XBr

_{2}+ Br_{2}---> XBr_{4}XBr

_{2}and Br_{2}react in a 1:1 molar ratioTherefore, 10.00 g represents 0.02722 mol of XBr

_{2}10.00 g / 0.02722 mol = 367 g/mol

In 367.377 g of XBr

_{2}, there is 159.808 g of bromine367.377 − 159.808 = 207.569 <--- this is the atomic weight of X

X is lead.

**Problem #33:** Exactly 4.32 g of oxygen gas was required to completely combust a 2.16 g sample of a mixture of methanol and ethanol:

(1) How many moles of ethanol are contained within the sample?

(2) What is the percentage by weight of methanol in the sample?

**Solution:**

1) Balanced chemical equations:

CH_{3}OH + 1.5O_{2}---> CO_{2}+ 2H_{2}O

C_{2}H_{5}OH + 3O_{2}---> 2CO_{2}+ 3H_{2}O

2) Let x = mass of methanol; let y = mass of ethanol

x + y = 2.16[(1.5) (x/32)] + [(3) (y/46)] = 4.32/32

3) Using x = 2.16 − y:

[(1.5) ((2.16-y)/32)] + [(3) (y/46)] = 0.135[(3.24 − 1.5y) / 32] + [3y / 46] = 0.135

4) Multiply each side by 32, then by 46:

(46) (3.24 − 1.5y) + (32) (3y) = 198.72149.04 − 69y + 96y = 198.72

27y = 49.68

y = 1.84 g

x = 0.32 g

5) Answers to (1) and (2):

moles ethanol ---> 1.84 g / 46 g/mol = 0.040 molpercent methanol ---> (0.32 g / 2.16 g) * 100 = 14.8%

**Problem #34:** A 3.41 g sample of a metallic element, M, reacts completely with 0.0158 mol of a gas, X_{2}, to form 4.53 g MX. What are the identities of M and X?

**Solution:**

2M + X_{2}---> 2MXWe know that M and X

_{2}react in a 2:1 molar ratio.Therefore, 0.0158 mol of X

_{2}reacts with twice that many moles of M, 0.0316 molWe can now determine the atomic weight of M:

3.41 g / 0.0316 mol = 107.9 g/mol

From the periodic table, we see that M is silver.

4.53 − 3.41 = 1.12 g <--- this is the mass of X in MX

We know that there is a 1:1 molar ratio between M and MX, therefore 0.0316 of MX was produced. Since we know the formula is MX, we know that 0.0316 mol of X is involved.

1.12 g / 0.0316 mol = 35.44 g/mol

This atomic weight is within experimental error for X to be chlorine.

**Problem #35:** When 2.3 moles of X reacts with 1.6 moles of Y, 71 grams of Z are produced. What is the molar mass of Z?

3X + 4Y ---> 5Z

This reaction has a 50% yield.

**Solution:**

1) Determine the limiting reagent:

X: 2.3 / 3 = 0.77

Y: 1.6 / 4 = 0.4Y is the limiting reagent.

2) How much Z is produced at 100% yield:

4 is to 5 as 1.6 is to xx = 2.0 mol

3) The reaction has only 50% yield, so 1.0 mole of Z was produced. The molar mass of Z is:

71 g / 1.0 mol = 71 g/mol

**Problem #36:** 20.0 mL of solution containing NaCl and KCl gave, on evaporation to dryness, 0.180 g of the mixed chlorides. 20.0 mL of the same solution gave 0.370 g of AgCl on treatment with a slight excess of the AgNO_{3} solution. Calculate, for the original solution, the mass per liter of both chlorides.

**Solution:**

1) Let x = mass NaCl and let y = mass KCl. from that, we get our first equation:

x + y = 0.180

2) Using AgCl, determine the moles of chloride ion in the solution:

moles AgCl ---> 0.370 g / 143.321 g/mol = 0.00258 mol0.00258 mol gives the moles of chloride based on this:

AgFor every 1 mole of AgCl that precipitated, there was 1 mole of chloride in solution.^{+}(aq) + Cl¯(aq) ---> AgCl(s)

3) We can now write our second equation:

x / 58.4428 g/mol + y / 74.553 = 0.00258x / 58.4428 ---> moles of NaCl

y / 74.553 ---> moles of KClBased on the 1:1 molar ratios between NaCl and Cl¯ (as well as KCl and Cl¯), we know that the two above divisions give the moles of chloride contributed from the NaCl and the KCl.

4) We will now substitute the first equation into the second and solve:

x = 0.180 − y[(0.180 − y) / 58.4428] + [y / 74.553] = 0.00258

13.4 − 74.553y + 58.4428y = 11.2

16.1y = 2.2

y = 0.137 g <--- mass KCl in 20.0 mL

x = 0.180 − 0.137 = 0.0430 g <--- mass NaCl in 20.0 mLmass KCl in 1 L ---> 0.137 g / 0.020 L = 6.85 g

mass NaCl in 1 L ---> 0.043 / 0.020 L = 2.15 g

**Problem #37:** You are given a mixture of three hydrated salts: Na_{2}CO_{3} **·** 10H_{2}O, MgSO_{4} **·** 7H_{2}O, and CuSO_{4} **·** 5H_{2}O. The total mass of the mixture is 12.123 grams. When the mixture is heated gently, the following two reactions occur:

Na_{2}CO_{3}·10H_{2}O(s) ---> Na_{2}CO_{3}·7H_{2}O(s) + 3H_{2}O(g)

MgSO_{4}·7H_{2}O(s) ---> MgSO_{4}·H_{2}O(s) + 6H_{2}O(g)

After these reactions are complete, the mass of the mixture has decreased to 9.049 grams. This mixture is then heated more strongly, and the following additional reactions occur:

Na_{2}CO_{3}·7H_{2}O(s) ---> Na_{2}CO_{3}(s) + 7H_{2}O(g)

MgSO_{4}·H_{2}O(s) ---> MgSO_{4}(s) + H_{2}O(g)

CuSO_{4}·5H_{2}O(s) ---> CuSO_{4}(s) + 5H_{2}O(g)

After this final heating, the mass of the mixture has decreased to 6.412 grams. From this information, calculate the masses of each of the three compounds in the original mixture.

**Solution:**

Comment: three simultaneous equations in three unknowns are required.

1) The first equation:

X + Y + Z = 12.123 gwhere:

X = the mass of Na

_{2}CO_{3}·10H_{2}O

Y = the mass of MgSO_{4}·7H_{2}O

Z = the mass of CuSO_{4}·5H_{2}O

2) The second equation is developed based on the information gained from the first heating. Here is the second equation:

(232.0916 / 286.136) (X) + (138.3808 / 246.4696) (Y) + Z = 9.049 g(232.0916 / 286.136) ---> this gravimetric factor is the decimal percent decrease in the original mass (the X) of the sodium carbonate decahydrate as it is changed to the heptahydrate. 232 is the molar mass of the heptahydrate and 286 is the molar mass of the decahydrate.

(138.3808 / 246.4696) ---> this gravimetric factor is the decimal percent decrease in the original mass (the Y) of magnesium heptahydrate as it is changed to the monohydrate. 246 is the molar mass of the heptahydrate and 138 is the molar mass of the monohydrate

There is no gravimetric factor for copper(II) sulfate pentahydrate since it did not lose any mass. It started with Z grams present and ended with Z grams present.

By the way, after the heating, the sample is now composed of Na

_{2}CO_{3}·7H_{2}O, MgSO_{4}·H_{2}O, and CuSO_{4}·5H_{2}O.

3) The third equation is developed based on the information gained from the second heating. Here is the third equation:

(105.988 / 286.136) (X) + (120.366 / 246.4696) (Y) + (159.607 / 249.681) (Z) = 6.412 gNotice that, in each gravimetric factor, the denominator is the molar mass of the original hydrate. This is because the 6.412 value references the entire loss of mass from the starting value of 12.123 g.

4) I'm going to rewrite the three equations, but I will use decimals rather than fractions:

X + Y + Z = 12.1230.8111234X + 0.5614518Y + Z = 9.049

0.3704113X + 0.4883604Y + 0.6392437Z = 6.412

5) To calculate the answers, I used a 3 Equation System Solver. When I did that, I got the following answers (which I rounded off):

1.373 grams of Na_{2}CO_{3}·10H_{2}O

6.418 grams of MgSO_{4}·7H_{2}O

4.332 grams of CuSO_{4}·5H_{2}O

6) I will start a step-by-step solution:

Rewrite the first equation:Z = 12.123 − (X + Y)Substitute into the second and third equations:

0.8111234X + 0.5614518Y + [12.123 − (X + Y)] = 9.0490.3704113X + 0.4883604Y + 0.6392437[12.123 − (X + Y)] = 6.412

We now have two simultaneous equations in 2 unknowns.

And that is where I will leave it.

7) I must admit that I did not solve this problem on my own. When I found the problem in my notes, I did an Internet search which yielded some discussion about this problem. Look for links to 'www.chemicalforums.com'

**Problem #38:** A mixture of CuSO_{4} **·** 5H_{2}O and MgSO_{4} **·** 7H_{2}O is heated until all the water is lost. If 5.020 g of the mixture gives 2.988 g of the anhydrous salts, what is the percent by mass of CuSO_{4} **·** 5H_{2}O in the mixture?

**Solution:**

1) For every mole of CuSO_{4} there are 5 moles of H_{2}O, and for every mole of MgSO_{4} there are 7 moles of H_{2}O. Therefore:

(5 * moles of CuSO_{4}) + (7 * moles of MgSO_{4}) = moles of H_{2}O

2) Determine moles of water lost:

5.020 − 2.988 = 2.032 g2.032 g / 18.015 g/mol = 0.112795 mol

3) Let X be the grams of anhydrous CuSO_{4}. Therefore:

2.988 − X ---> the grams of anhydrous MgSO_{4}

3) We can now substitute into the equation in step 1:

5 * (X / 159.6096) + 7 * [(2.988 − X) / 120.3686] = 0.112795X / 159.6096 ---> moles of anhydrous CuSO

_{4}

(2.988 − X) / 120.3686 ---> moles of anhydrous MgSO_{4}

4) Solve it:

(159.6096) (120.3686) * 5 * (X / 159.6096) + (159.6096) (120.3686) * 7 * [(2.988 − X) / 120.3686] = (0.112795) (159.6096) (120.3686)(120.3686) * 5 * X + (159.6096) * 7 * (2.988 − X) = 2167.016

601.843X + 3338.3944 − 1117.2672X = 2167.016

515.4242X = 1171.3784

x = 2.27265 g of anhydrous CuSO

_{4}

5) Determine moles of anhydrous CuSO_{4}:

2.27265 g / 159.6096 g/mol = 0.0142388 mol

6) Determine mass of water associated with 0.0142388 mol of anhydrous CuSO_{4}:

(0.0142388 mol) (5) = 0.071194 mol of H_{2}O(0.071194 mol) (18.015 g/mol) = 1.28256 g

7) Determine mass of CuSO_{4} **·** 5H_{2}O in the original sample and its percentage:

2.27265 g + 1.28256 g = 3.55521 g(3.55521 / 5.020) * 100 = 70.82%

**Problem #39:** One mole of hydrocarbon Z is subjected to combustion. The product obtained is condensed and the resulting gaseous product occupied a volume of 89.6 L at STP. Oxygen required for this combustion was 145.6 L at STP. What is the molecular formula of Z?

**Solution:**

1) Let the formula for Z be this:

C_{x}H_{y}

2) Determine moles of CO_{2} and O_{2}:

89.6 L / 22.4 L/mol = 4.00 mol CO_{2}145.6 L / 22.4 L/mol = 6.50 mol O

_{2}

3) The reaction thus far:

C_{x}H_{y}+ 6.5O_{2}---> 4CO_{2}+ ___H_{2}O

4) Determine the coefficient for H_{2}O:

6.50 − 4.00 = 2.50 mol of O_{2}in the H_{2}Ogiving 5.00 mol H

_{2}O

5) Therefore:

C_{x}H_{y}+ 6.5O_{2}---> 4CO_{2}+ 5H_{2}Ox must be 4 and y must be 10

C

_{4}H_{10}

**Problem #40:** A sample contains BaCl_{2} **⋅** 2H_{2}O and NaCl. After 0.678 grams of the sample was heated, the residue weighed 0.648 grams. What is the percent of BaCl_{2} **⋅**2H_{2}O in the sample?

**Solution:**

1) Determine water driven off:

0.678 g − 0.648 g = 0.030 g H_{2}O

2) Determine moles of water driven off:

0.030 g / 18.015 g / mol = 0.00166528 mol H_{2}O

3) Determine moles of barium chloride dihydrate in sample:

(0.00166528 mol H_{2}O) (1 mol BaCl_{2}⋅2H_{2}O / 2 mol H_{2}O) = 0.00083264 mol BaCl_{2}⋅2H_{2}O

4) Determine grams of BaCl_{2} **⋅** 2H_{2}O:

(0.00083264 mol) ( 244.2656 g / mol) = 0.2034 g

5) Determine percent BaCl_{2} **⋅** 2H_{2}O in sample:

(0.2034 g / 0.678 g) * 100 = 30.0%

**Problem #41:** A 10.00 g sample of a mixture of CaCl_{2} and NaCl is treated with Na_{2}CO_{3} to precipitate the CaCO_{3}. This CaCO_{3} is heated to convert all the calcium to CaO and the final mass of CaO is 1.620 g. Determine the percent by mass of CaCl_{2} in the original mixture.

**Discussion:** We are given the mass of CaO, which will be used to calculate the mass of CaCO_{3} produced. Once we know the CaCO_{3} amount, we can determine the amount of CaCl_{2} present in the original mixture.

**Solution:**

1) When the mixture is heated, only this reaction occurs:

CaCO_{3}+ heat ---> CaO + CO_{2}This balanced equation will be used as a math platform to show the needed calculation.

2) Over the CaO, enter 1.620 g and below the same, enter its gram-formula weight of 56.077 g/mol:

1.620 CaCO _{3}---> CaO + CO _{2}56.077

3) Over the CaCO_{3}, enter X and below the same, enter its gram-formula weight of l00.086 g/mol:

X 1.620 CaCO _{3}---> CaO + CO _{2}l00.086 56.077

4) Cross multiply through the equation and solve for X:

X 1.620 g ––––––––––– = ––––––––––– 100.086 g/mol 56.077 g/mol X = 2.89137 g CaCO

_{3}(I kept some guard digits)

5) We turn our attention to the equation that produces the calcium carbonate:

CaCl_{2}+ Na_{2}CO_{3}---> CaCO_{3}+ 2NaCl

6) Over the CaCO_{3}, enter 2.89137 g and below the same, enter its gram-formula weight of l00.086 g/mol:

2.89137 CaCl _{2}+ Na _{2}CO_{3}---> CaCO _{3}+ 2NaCl 100.086

7) Over the CaCl_{2}, enter X and, below the same, enter its gram-formula weight of 110.984 g/mol

X 2.89137 CaCl _{2}+ Na _{2}CO_{3}---> CaCO _{3}+ 2NaCl 110.984 100.086

8) Cross multiply through the equation and solve for X:

X 2.89137 g ––––––––––– = ––––––––––– 110.984 g/mol 100.086 g/mol X = 3.2062 g CaCl

_{2}(I kept some guard digits)

9) Percent CaCl_{2} in original mixture:

(3.2062 g / l0.00 g) * 100 = 32.06% (o 4 sig figs)

**Problem #42:** How many milliliters of sodium metal, with a density of 0.971 g/mL, would be needed to produce 8.75 grams of hydrogen gas in a single replacement reaction with HCl?

**Solution:**

1) Write the balanced chemical equation:

Na + HCl ---> NaCl + 0.5H_{2}<--- note the 0.5 coefficient for the hydrogen gas

2) Determine moles of hydrogen gas produced:

8.75 g ––––––––– = 4.340278 mol 2.016 g/mol

3) Use a Na to H_{2} mole ratio to determine moles of sodium that reacted:

1 x –––– = ––––––––––– 0.5 4.340278 mol x = 8.680556 mol Na consumed

4) Determine grams of Na in 8.680556 mol Na:

(8.680556 mol) (22.9898 g/mol) = 199.564 g

5) Determine volume of the sodium

199.564 g ––––––––– = 205.524 mL 0.971g/mL To three sig figs, the answer is 206 mL

**Problem #43:** A 5.75 g mixture contains both lithium fluoride, LiF, and potassium fluoride, KF. If the mixture contains 3.42 g fluorine, what is the mass of the KF in the mixture?

**Solution:**

1) Let "Z" be the mass (in grams) of the KF in the mixture.

2) Let (5.75 − Z) is the mass of LiF in the mixture.

3) Write expressions for the mass of F in each of the two compounds in the mixture.

Z 1 mol KF 1 mol F 18.9984 g F ––– x ––––––––––– x ––––––– x ––––––––– = (0.327013Z) g F per g KF 1 58.0967 g KF 1 mol KF 1 mol F

5.75 − Z 1 mol LiF 1 mol F 18.9984 g F ––––––– x ––––––––––– x ––––––– x ––––––––– = (4.21139 − 0.732415Z) g F per g LiF 1 25.9394 g LiF 1 mol LiF 1 mol F

4) Set the sum of the two expressions for grams of F equal to the given total grams of F (all units are in grams):

(0.327013Z) + (4.21139 − 0.732415Z) = 3.42Z = 1.95 g KF

**Problem #44:** A mixture of two hydrocarbons, C_{8}H_{18} (octane) and C_{7}H_{8} (toluene), has a mass of 191 g. The hydrocarbon mixture is burned in excess oxygen to form a mixture of carbon dioxide and water that contains 1.56 times as many moles of carbon dioxide as water. Find the masses of C_{8}H_{18} and C_{7}H_{8} in the mixture.

**Solution:**

1) Let "Z" be the mass in grams of C_{8}H_{18}.

2) Let (191 − z) grams be the mass of C_{7}H_{8}.

3) Write the balanced chemical equation for the combustion of octane:

2C_{8}H_{18}+ 25O_{2}---> 16CO_{2}+ 18H_{2}O

4) Write expressions for the molar amount of CO_{2} and H_{2}O produced from combusting C_{8}H_{18}.

Z 1 mol C _{8}H_{18}16 mol CO _{2}––– x –––––––––––––– x –––––––––– = (0.0700351Z) mol CO _{2}from C_{8}H_{18}1 114.2285 g C _{8}H_{18}2 mol C _{8}H_{18}

Z 1 mol C _{8}H_{18}18 mol H _{2}O––– x –––––––––––––– x –––––––––– = (0.0787894Z) mol H _{2}O from C_{8}H_{18}1 114.2285 g C _{8}H_{18}2 mol C _{8}H_{18}

5) Write the balanced chemical equation for the combustion of C_{7}H_{8}.

C_{7}H_{8}+ 9O_{2}---> 7CO_{2}+ 4H_{2}O

6) Write expressions for the molar amount of CO_{2} and H_{2}O produced from combusting C_{7}H_{8}.

191 − Z 1 mol C _{7}H_{8}7 mol CO _{2}––––––– x –––––––––––– x –––––––––– = (14.5108 − 0.075973Z) mol CO _{2}from C_{7}H_{8}1 92.1384 g C _{7}H_{8}1 mol C _{7}H_{8}

191 − Z 1 mol C _{7}H_{8}4 mol H _{2}O––––––– x –––––––––––– x –––––––––– = (8.29187 − 0.043413Z) mol H _{2}O from C_{7}H_{8}1 92.1384 g C _{7}H_{8}1 mol C _{7}H_{8}

7) Add the two sources of CO_{2} and of H_{2}O together:

total mol of CO_{2}---> (0.0700351Z) + (14.5108 − 0.075973Z) = (14.5108 − 0.0059379Z)total mol of H

_{2}O ---> (0.0787894Z) + (8.29187 − 0.043413Z) = (8.29187 + 0.0353764Z)

8) Write the expression for the ratio of total moles of CO_{2} to total moles of H_{2}O, then set it equal to the ratio given in the problem statement:

14.5108 − 0.0059379Z 1.56 ––––––––––––––––––– = –––– 8.29187 + 0.0353764Z 1 Z ≈ 25.8 g C

_{8}H_{18}(191 − Z) = 165 g C

_{7}H_{8}(to 3 sig figs)

**Problem #45:** 20 liters of SO_{2} reacts completely with 40 liters O_{2} to form SO_{3}. At the end of the reaction, the combined volume of the substances is?

**Solution:**

1) Write the balanced chemical equation:

2SO_{2}+ O_{2}---> 2SO_{3}

2) No information about temperature and pressure is provided. We, therefore, will assume that the volumes of the reactant (and products) are all at the same temperature and the same pressure.

3) The above allows us to invoke Avogadro's Hypothesis:

Equal volumes of gas, at equal temperatures and pressures, contain equal number of molecules

4) This means that the coefficients of a balanced chemical equation can be interpreted as volumes (a more general word than liters). Using the reaction in this problem:

two volumes of SO_{2}react with one volume of O_{2}to produce two volumes of SO_{3}

5) The SO_{2} to O_{2} volume ratio is 2:1. Therefore:

20 L of SO_{2}react with 10 L of O_{2}(remember, 2:1 ratio)The SO

_{2}is completely consumed and 30 L of O_{2}remain.

6) The SO_{2} to SO_{3} ratio is 2:2. Therefore:

The 20 L of SO_{2}that reacted (and was completely consumed, making it the limiting reagent) will produce 20 L of SO_{3}

7) 30 L of O_{2} left over and 20 litres of SO_{3} produced by the reaction means 50 L total.

**Problem #46:** How many NaBr formula units are formed when 50 NBr_{3} molecules and 57 NaOH formula units react? (Formula units are the equivalent of molecules for ionic compounds.)

2NBr_{3} + 3NaOH ---> N_{2} + 3NaBr + 3HBrO

**Solution #1:**

Of the two reactants, which is excess and which is the limiting one? The ratio is 2:3 and you have 50:57, so the 57 NaOH formula units is the limiting factor, it would use 2/3 of 57 = 38 NBr_{3}molecules.Ratio of NaBr to NaOH is 3:3 or 1:1, so 57 NaOH formula units form 57 NaBr formula units.

**Solution #2:**

number of NaOH required = (3/2) * number of NBr_{3}(3/2) * 50 = 75

but we have just 57 NaOH. So, NaOH is limiting reagent

we will use NaOH in further calculation

from 1:1 ratio in reaction,

number of NaBr formed = number of NaOH = 57

**Problem #47:** According to the label on a bottle of concentrated hydrochloric acid, the contents are 36.0% by mass and has a density of 1.442 g/mL. What mass in kg of sodium hydrogen carbonate is needed to neutralize the spill if a bottle containing 5.127 L of HCl dropped and broke open.

**Solution:**

1) Balanced reaction (always a great place to start)

HCl + NaHCO_{3}---> NaCl + H_{2}O + CO_{2}Note the 1:1 molar ratio of HCl and NaHCO

_{3}

2) Big dimensional analysis set up (I will use 'sol' to indicate the HCl solution):

5.127 L sol 1000 mL sol 1.422 g sol 36.0 g HCl 1 mol HCl 1 mol NaHCO _{3}84.007 g NaHCO _{3}1 kg ––––––– x ––––––– x ––––––– x ––––––– x ––––––– x ––––––– x ––––––– x ––––––– = 6.05 kg 1 1 L sol 1 mL sol 100 g sol 36.4609 g HCl 1 mol HCl 1 mol NaHCO _{3}1000 g

**Problem #48:** An ore containing magnetic, Fe_{3}O_{4},was anlyzed by dissolving a 1.5419 g sample in concentrated HCI , giving a mixture of Fe^{2+} and Fe^{3+}.

After adding HNO_{3} to oxidize any Fe^{2+} to Fe^{3+}, the resulting solution was diluted with water and the Fe^{3+} precipitated as Fe(OH)_{3} by adding NH_{3}. After filtering and rinsing, the residue was ignited , giving 0.8525 g of pure Fe_{2}O_{3}.

Calculate the % w/w Fe_{3}O_{4} and % w/w Fe in the sample.

**Solution:**

1) All of the iron atoms in the final residue came from the original ore sample, regardless of all the intermediate reactions.

2) Calculate mass percent of Fe_{3}O_{4}:

0.8525 g Fe _{2}O_{3}1 mol Fe _{2}O_{3}2 mol Fe 1 mol Fe _{3}O_{4}231.53 g Fe _{3}O_{4}1 –––––––––––– x –––––––––––– x –––––––––– x –––––––––– x –––––––––––– x ––––––– = 0.5346 = 53.46% Fe _{3}O_{4}1 159.64 g Fe _{2}O_{3}1 mol Fe _{2}O_{3}3 mol Fe 1 mol Fe _{3}O_{4}1.5419 g

3) Calculate mass percent of Fe:

0.8525 g Fe _{2}O_{3}1 mol Fe _{2}O_{3}2 mol Fe 55.85 g Fe 1 –––––––––––– x –––––––––––– x –––––––––– x –––––––––– x –––––––––––– = 0.3868 = 38.68% Fe 1 159.64 g Fe _{2}O_{3}1 mol Fe _{2}O_{3}1 mol Fe 1.5419 g

**Problem #49:** Potassium thiosulfate, K_{2}S_{2}O_{3}, is used to remove any excess chlorine from fibers and fabrics that have been bleached with that gas.

K_{2}S_{2}O_{3}+ 4Cl_{2}+ 5H_{2}O ---> 2KHSO_{4}+ 8HCl

(a) How many moles of K_{2}S_{2}O_{3} must react to produce 2.803 g of HCl?

(b) How many grams of Cl_{2} must react to produce 21.27 g of KHSO_{4}?

(c) How many molecules of HCl are produced at the same time that 2.513 g of KHSO_{4} is produced?

**Solution to (a):**

2.803 g HCl 1 mol HCl 1 mol K _{2}S_{2}O_{3}–––––––––– x ––––––––––– x ––––––––––– = 0.00961 mol K _{2}S_{2}O_{3}1 36.4609 g HCl 8 mol HCl

**Solution to (b):**

21.27 g KHSO _{4}1 mol KHSO _{4}4 mol Cl _{2}70.906 g Cl _{2}––––––––––––– x ––––––––––––––– x ––––––––––– x –––––––––– = 22.15 g Cl _{2}1 136.167 g KHSO _{4}2 mol KHSO _{4}1 mol Cl _{2}

**Solution to (c):**

2.513 g KHSO _{4}1 mol KHSO _{4}8 mol HCl 6.022 x 10 ^{23}molecules HCl––––––––––––– x ––––––––––––– x –––––––––– x –––––––––––––––––––––– = 4.446 x 10 ^{22}molecules HCl1 136.167 g KHSO _{4}2 mol KHSO _{4}1 mol HCl

**Problem #50:** Calculate minimum mass of P_{4}S_{3} is required to produce at least 1.00 gm of each product.

P_{4}S_{3}+ 8O_{2}---> P_{4}O_{10}+ 3SO_{2}

**Discussion:**

This problem is solved by doing two mass-mass problems involving P_{4}S_{3} in both and paired with each of the two products in two calculations. The greater amount of P_{4}S_{3} will be the answer to the problem.

**Solution:**

1) How much P_{4}S_{3} required to produce 1.00 g of P_{4}O_{10}?

1.00 g P _{4}O_{10}1 mol P _{4}O_{10}1 mol P _{4}S_{3}220.091 g P _{4}S_{3}––––––––––– x ––––––––––––– x –––––––––– x –––––––––––– = 0.775 g (to three sig figs) 1 283.886 g P _{4}O_{10}1 mol P _{4}O_{10}1 mol P _{4}S_{3}

2) How much P_{4}S_{3} required to produce 1.00 g of SO_{2}?

1.00 g SO _{2}1 mol SO _{2}1 mol P _{4}S_{3}220.091 g P _{4}S_{3}––––––––– x –––––––––– x –––––––––– x –––––––––––– = 1.145 g (to four sig figs) 1 64.063 g SO _{2}3 mol SO _{2}1 mol P _{4}S_{3}

3) To three sig figs, the answer to this problem is 1.14 g P_{4}S_{3}.