Stoichiometry - AP level
All Examples and Problems

Problems #1-10

Problems #11-25

Problems #26-50

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Ten Examples

Example #1: A mixture of Mg and Zn with a combined mass of 1.0875 g was burned in oxygen producing MgO and ZnO with a combined mass of 1.4090 g. How many grams of zinc was in original mixture?

Example #2: A metal container of a fixed volume contains water vapor at 200. °C at a pressure of 4.50 atm. The water is then electrolytically split to generate hydrogen (H2) and oxygen (O2) gases. What will be the pressure (in atm) of the container at 200. °C after the water has been completely split?

Example #3: A metal container of fixed volume is filled with 2.50 atm of H2 gas and 2.50 atm of O2 at 200. °C. The gases then react to generate water vapor (H2O gas) through a combustion reaction. What is the total pressure (in atm) in the container at 200. °C when the reaction has progressed to the point where it has produced the maximum amount of H2O gas?

Example #4: 1 mole of hydrocarbon of formula CnH2n was burned completely in oxygen producing carbon dioxide and water vapour only. It required 192 grams of oxygen. Determine the formula of the hydrocarbon.

Example #5: A 13.43 g mixture of CH4 and C2H6 is completely burned in excess oxygen. The mixture of CO2 and H2O that results weighs 64.84 g. Determine the fraction of CH4 in the original mixture.

Example #6: You are given 1.446 g of a mixture of KClO3 and KCl. When heated, the KClO3 decomposes to KCl and O2:

2KClO3(s) ---> 2KCl(s) + 3O2(g)

and 355 mL of O2 is collected over water at 26.0 °C. The total pressure of the gases in the collection flask is 745.0 torr. What is the weight percentage of KClO3 in the sample?

Example #7: A mixture of potassium chloride and potassium bromide weighing 3.595 g is heated with chlorine gas, which converts the mixture completely to potassium chloride. The total mass of the KCl after the reaction is 3.129 g. What percent of the original mixture is KBr?

Example #8: An alloy contain aluminum, copper, and zinc. You take a 12.657 g sample of the alloy and react it with an HCl solution. The reaction converts all of the aluminum and zinc in the alloy to aluminum chloride and zinc chloride in addition to producing hydrogen gas. The copper does not react with the HCl. Upon completion of the reaction, a total of 10.2 L of hydrogen gas was collected at a pressure of 726 torr and a temperature of 27.0 °C. Additionally, 2.642 g of unreacted copper is recovered. What is the percent composition of the alloy?

Example #9: A sample of sodium bicarbonate (NaHCO3) is known to contain some impurities. It is found that Na makes up 18.00% of the entire mass of the sample. All of the Na comes from the NaHCO3. Find the mass percent of NaHCO3 in the sample.

Example #10: An ore of barium contains BaCO3. A 1.495 g sample of the ore was treated with HCl to dissolve the BaCO3. The resulting solution was filtered to remove insoluble material and then treated with H2SO4 to precipitate BaSO4. The precipitate was filtered, dried, and found to weigh 1.152 g. What is the percentage by mass of barium in the original sample?


Problems #1-10

Problem #1: A salt contains only barium and one of the halide ions. A 0.1480 g sample of the salt was dissolved in water and an excess of sulfuric acid was added to form barium sulfate, which was filtered, dried and weighed. Its mass was found to be 0.1660 g. What is the formula for the barium halide?

Problem #2: A 4.000 g sample of M2S3 is converted to MO2 and loses 0.277 g. What is the atomic weight of M?

Problem #3a: A 5.000 gram sample of a dry mixture of potassium hydroxide, potassium carbonate and potassium chloride is reacted with 0.100 L of 2.00-molar HCl solution. A 249.0 mL sample of dry carbon dioxide gas, measured at 22.0 °C and 740.0 torr, is obtained from this reaction. What was the percentage of potassium carbonate in the mixture?

Problem #3b: The excess HCl in problem 3a was found by titration to be chemically equivalent to 86.60 mL of 1.50-molar sodium hydroxide. What was the percentage of KOH and of KCl in the original mixture?

Problem #4a: For the reaction below, when 0.5000 g of XI3 reacts completely, 0.2360 g of XCl3 is obtained. Calculate the atomic weight of element X and identify it.

Problem #4b: If 0.520 grams of XCl3 are treated with iodine, 0.979 g of XI3 are produced. What is the chemical symbol for this element?

Problem #5: A 2.077 g sample of an element, which has an atomic mass between 40 and 55, reacts with oxygen to form 3.708 g of an oxide. Determine the formula mass of the oxide (and identify the element).

Problem #6: A 12.5843 g sample of ZrBr4 was dissolved and, after several steps, all of the combined bromine was precipitated as AgBr. The silver content of AgBr was found to be 13.2160 g. Assume the atomic masses of silver and bromine to be 107.868 and 79.904. What value was obtained for the atomic mass of Zr from this experiment?

Problem #7: Two different chloride compounds of platinum are known, compound X and Y. When 3.45 g of compound X is heated, 2.72 g of compound Y is formed along with some chlorine gas. Upon further heating, the 2.72 g of compound Y is decomposed to 1.99 g of platinum metal and some more chlorine gas. Determine the formulas of compounds X and Y.

Problem #8: The active ingredients of an antacid tablet contained only magnesium hydroxide and aluminum hydroxide. Complete neutralization of a sample of the active ingredients required 48.5 mL of 0.187 M hydrochloric acid. The chloride salts from this neutralization were obtained by evaporation of the filtrate from the titration; they weighed 0.4200 g. What was the percentage by mass of magnesium hydroxide in the active ingredients of the antacid tablet?

Problem #9: When the supply of oxygen is limited, iron metal reacts with oxygen to produce a mixture of FeO and Fe2O3. In a certain experiment, 20.00 g of iron metal was reacted with 11.20 g of oxygen gas. After the experiment the iron was totally consumed and 3.56 g oxygen gas remained. Calculate the amounts of FeO and Fe2O3 formed in this experiment.

Problem #10: 0.197 g of magnesium is burned in air:

2Mg + O2 ---> 2MgO

However, some of the magnesium reacts with nitrogen in the air to form magnesium nitride instead:

3Mg + N2 ---> Mg3N2

So you have a mixture of MgO and Mg3N2 weighing 0.315 g. Determine what percentage of the Mg formed the nitride in the initial reaction.


Problems #11-25

Problem #11: Hydroxylammonium chloride reacts with iron(III) chloride, FeCl3, in solution to produce iron(II) chloride, HCl, H2O and a compound of nitrogen. It was found that 2.00 g of iron(III) chloride reacted in this way with 31.0 mL of 0.200 M hydroxylammonium chloride. Suggest a possible formula for the compound of nitrogen so produced.

Problem #12: How many phosphate ions are in a sample of hydroxyapatite [Ca5(PO4)3OH] that contains 5.50 x 10-3 grams of oxygen?

Problem #13: A mixture consisting of only sodium chloride (NaCl) and potassium chloride (KCl) weighs 1.0000 g. When the mixture is dissolved in water and an excess of silver nitrate is added, all the chloride ions associated with the original mixture are precipitated as insoluble silver chloride (AgCl). The mass of the silver chloride is found to be 2.1476 g. Calculate the mass percentages of sodium chloride and potassium chloride in the original mixture.

Problem #14: Ammonia is produce industrially by reacting:

N2 + 3H2 ---> 2NH3

Assuming 100% yield, what mass of ammonia will be produced from a 1:1 molar ratio mixture in a reactor that has a volume of 8.75 x 103 L under a total pressure of 2.75 x 107 Pa at 455 °C.

Problem #15: Upon heating, a 4.250 g sample loses 0.314 grams. Assuming the sample is BaCl2 · 2H2O and NaCl, calculate the mass percent of BaCl2 · 2H2O.

Problem #16: A 0.6118 g sample containing only MgCl2 and NaCl was analyzed by adding 145.0 mL of 0.1006 M AgNO3. The precipitate of AgCl(s) formed had a mass of 1.7272 g. Calculate the mass of each component (MgCl2 and NaCl) in the original sample.

Problem #17: Ammonium nitrate and potassium chlorate both produce oxygen gas when decomposed by heating. Without doing detailed calculations, determine which of the two yields the greater

(a) number of moles of O2 per mole of solid and
(b) number of grams of O2 per gram of solid.

The unbalanced equations are:

NH4NO3(s) ---> N2(g) + O2(g) + H2O
KClO3(s) ---> KCl(s) + O2(g)

Problem #18: An element X forms both a dichloride (XCl2) and a tetrachloride (XCl4), Treatment of 10.00 g XCl2 with excess chlorine forms 12.55 g XCl4. Calculate the atomic mass of X, and identify X.

Problem #19: Water is added to 4.267 g of UF6. The only products of the reaction are 3.730 g of a solid containg only uranium, oxygen, and fluorine and 0.970 g of a gas. The gas is 95.0% fluorine and the remainder is hydrogen.

a) What fraction of the fluorine of the orginal is in the solid and what fraction in the gas after the reaction?
b) What is the formula of the solid product?

Problem #20: A compound containing titanium and chlorine is analyzed by converting all the titanium into 1.20 g of titanium dioxide and all the chlorine into 6.45 g of AgCl. What is the simplest (empirical) formula for the original compound?

Problem #21: An unknown element X is found in two compounds, XCl2 and XBr2. In the following reaction:

XBr2 + Cl2 ---> XCl2 + Br2

when 1.5000 g XBr2 is used, 0.8897 g XCl2 is formed. Identify the element X.

Problem #22: When the supply of oxygen is limited, iron metal reacts with oxygen to produce a mixture of FeO and Fe2O3. In a certain experiment, 20.00 g of iron metal was reacted with 11.20 g of oxygen gas. After the experiment the iron was totally consumed and 3.24 g oxygen gas remained. Calculate the amounts of FeO and Fe2O3 formed in this experiment.

Problem #23: A sheet of iron with a surface area of 525 cm2 is covered with a coating of rust that has an average thickness of 0.0021 cm. What minimum volume of an HCl solution, in mL, having a density of 1.07 g/mL and consisting of 14% HCl by mass is required to clean the surface of the metal by reacting with the rust? Assume that the rust is Fe2O3(s), that it has a density of 5.2 g/cm3, and that the reaction is:

Fe2O3(s) + 6HCl(aq) ---> 2FeCl3(aq) + 3H2O(ℓ)

Problem #24: A 1.42 g sample of a pure compound, with formula M2SO4, was dissolved in water and treated with an excess of aqueous calcium chloride, resulting in the precipitation of all the sulfate ions as calcium sulfate. The precipitate was collected, dried, and found to weigh 1.36 g. Determine the atomic mass of M. What element is it?

Problem #25: Calculate the volume change when iron is oxidized to Fe2O3 (d = 5.24 g/cm3). The density of Fe is 7.787 g/cm3.


Problems #26-50

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

M + H2SO4 ---> MSO4 + H2

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.

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) + CO2(g)

If 5.00 g of an unknown metal oxide (MO) reacted with excess carbon and formed 738 mL of CO2 at 200.0 °C and 0.978 atm, what is the identity of the metal?

Problem #28: A compound of P and F was analyzed as follows: heating 0.2324 g of the compound in a 378 cm3 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 CaF2. Determine the molecular formula of the compound.

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

MCl2 + 2AgNO3 ---> M(NO3)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.

Problem #30: An unidentified metal M reacts with an unidentified halogen X to form a compound MX2. When heated the compound decomposes by the reaction:

2MX2 (s) ---> 2MX (s) + X2 (g)

When 1.12 g of MX2 is heated, 0.720 g of MX is obtained along with 56.0 mL of X2 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?

Problem #31: A metal sulfate has the formula M2SO4. 10.99 g of the compound was dissolved in water to make 500.0 cm3 of solution. A 25.0 cm3 sample was removed and reacted with an excess of BaCl2(aq) to produce a precipitate of BaSO4, which when dried had a mass of 1.167 g.

a) Determine the number of moles of BaSO4 precipitated.
b) Determine the concentration of M2SO4
c) Identify M

Problem #32: An element, X, forms two compounds with bromine: XBr2 and XBr4. When 10.00 grams of the XBr2 is reacted with excess bromine, 14.35 g of XBr4 is formed. Identify X.

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?

Problem #34: A 3.41 g sample of a metallic element, M, reacts completely with 0.0158 mol of a gas, X2, to form 4.53 g MX. What are the identities of M and X?

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.

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 AgNO3 solution. Calculate, for the original solution, the mass per liter of both chlorides.

Problem #37: You are given a mixture of three hydrated salts: Na2CO3 · 10H2O, MgSO4 · 7H2O, and CuSO4 · 5H2O. The total mass of the mixture is 12.123 grams. When the mixture is heated gently, the following two reactions occur:

Na2CO3 · 10H2O(s) ---> Na2CO3 · 7H2O(s) + 3H2O(g)
MgSO4 · 7H2O(s) ---> MgSO4 · H2O(s) + 6H2O(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:

Na2CO3 · 7H2O(s) ---> Na2CO3(s) + 7H2O(g)
MgSO4 · H2O(s) ---> MgSO4(s) + H2O(g)
CuSO4 · 5H2O(s) ---> CuSO4(s) + 5H2O(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.

Problem #38: A mixture of CuSO4 · 5H2O and MgSO4 · 7H2O 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 CuSO4 · 5H2O in the mixture?


Problems #1-10

Problems #11-25

Problems #26-50

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