Ten Examples

Problems #1 - 10 | Return to Mole Table of Contents |

Determine empirical formula when given mass data

Determine empirical formula when given percent composition data

Determine identity of an element from a binary formula and mass data

Determine molecular formula using the Ideal Gas Law

Determine the formula of a hydrate Hydrate lab calculations

Examples and Problems only, no solutions

**Example #1:** A metal (M) forms an oxide with the formula MO. If the oxide contains 39.70% O by mass, what is the identity of M?

**Solution:**

1) Assme 100 g of the compound is present:

39.70 g is O; 60.30 g is M

2) Calculate moles of oxygen:

39.70 g / 16.00 g/mol = 2.48 mol

3) Determine moles of M:

The M to O molar ratio is 1:12.48 is to 1 as x is to 1

x = 2.48 mol of M

4) Calculate atomic weight of M and identify it:

60.30 g / 2.48 mol = 24.314 g/molM is magnesium

**Example #2:** A certain metal sulfide, MS_{2}, is determined to be 40.064% sulfur by mass. What is the identity of the metal M?

**Solution #1:**

1) Assume 100 g of the compound is present. Therefore:

40.064 g is sulfur; 59.936 g is M

2) Calculate moles of sulfur:

40.064 g / 32.065 g/mol = 1.249462 mol

3) Use molar ratio to get moles of M:

the M to S molar ratio is 1 : 2x is to 1 as 1.249462 mol is to 2

x = 0.624731 mol of M

4) Determine atomic weight of M:

59.936 g / 0.624731 mol = 95.94 g/molM is molybdenum

**Solution #2:**

1) Determine molar mass of MS_{2}

64.130 is to 0.40064 as x is to 1x = 160.069 g/mol

2) Subtract weight of sulfur:

160.069 − 64.130 = 95.94 g/mol

N.B. the 64.130 value is the weight of two sulfurs.

**Example #3:** A compound is found to have the formula XBr_{2}, in which X is an unknown element. Bromine is found to be 71.55% of the compound. Determine the identity of X.

**Solution #1:**

1) Determine molar mass of XBr_{2}

159.808 is to 0.7155 as x is to 1x = 223.3515 g/mol

2) Subtract weight of the two bromines:

223.3515 − 159.808 = 63.543 g/molThe element is copper.

**Solution #2:**

Let us assume 100 g of the compound is present. That means 71.55 g of Br is in the compound.71.55 g / 79.904 g = 0.89545 mol

0.89545 mol is to 2 as y is to 1

y = 0.447725 mole of X is present in 100 g of XBr

_{2}100 g − 71.55 g = 28.45 g of X in 100 g of XBr

_{2}28.45 g / 0.447725 = 63.54 g/mol

X is copper

**Example #4:** A compound whose empirical formula is XF_{3} consists of 64.79% F by mass. What is the atomic mass of X? What element is it?

**Solution:**

Let us assume 100 g of XF_{3}is present.64.79 g of it is F and 35.21 g of it is X

64.79 g / 18.9984 g/mol = 3.4103 mol

The molar ratio of X to F is 1 to 3

1 is to 3 as y is to 3.4103 mol

y = 1.13677 mol (this is how many moles of X are in the 100 g of XF

_{3})35.21 g / 1.13677 mol = 30.97 g/mol

X is phosphorous.

**Example #5:** An element X forms the pentabromide XBr_{5}. Analysis of XBr_{5} shows that it contains 92.81% Br by mass. What is element X?

**Solution:**

1) Let us assume 100 g of XBr_{5} is present. That means this:

92.81 g of Br is present and 7.19 g of X

2) Calculate moles of Br:

92.81 g / 79.9 g/mol = 1.1616 mol

3) Determine moles of X:

1.1616 mol is to 5 as x is to 1x = 0.23232 mol

4) Determine atomic weight of X:

7.19 g / 0.23232 mol = 30.95 g/molThe element is phosphorous.

**Example #6:** An element X forms the tetrachloride XCl_{4}. Analysis of XCl_{4} shows that it contains 40.63% Cl by mass. Identify X.

**Solution:**

1) Assume 100 g of the compound is present. This means:

40.63 g of chloride in the 100 g and 59.37 g of X

2) Determine moles of chloride:

40.63 g / 35.453 g/mol = 1.146 mol

3) Determine moles of X:

1.146 mol is to 4 as x is 1x = 0.2865 mol of X is present

4) Determine atomic weight of X:

59.37 g / 0.2865 mol = 207.2 g/molX is lead, PbCl

_{4}

**Example #7:** A metal M forms an oxide M_{2}O_{3} that contains 68.4% of the metal by mass. Calculate the atomic weight of the metal.

**Solution:**

68.4 is to 2x as 31.6 is to 48The 48 comes from 16 times 3 because there are 3 oxygens. It is 2x because there are two atoms of M.

x = 51.95, the atomic weight of chromium.

**Example #8:** A certain metal hydroxide, M(OH)_{2}, contains 32.80% oxygen by mass. What is the identity of the metal M?

**Solution #1:**

1) Write an expression for the molar mass of the compound:

Let M = the molar mass of the metal.M + (2 x 16.00) + (2 x 1.01)

M + 34.02

2) Write an expression for the given mass percent of oxygen:

[32.00 / (M + 34.02)] x 100% = 32.8%32.00 / (M + 34.02) = 0.328

3) Algebra!

32.00 = (0.3280)(M + 34.02)32.00 = 0.3280M + 11.16

20.84 = 0.3280M

M = 63.54

Copper.

**Solution #2:**

1) Determine mass percent of hydrogen:

32.80 is to 32.00 as x is to 2.02x = 2.07%

2) Determine mass percent of M:

100% − (32.80 + 2.07) = 65.13%

3) Determine atomic weight of M:

ONE metal atom has (65.13/32.80) times the weight of TWO oxygen atoms.the mass of the metal atom = (65.13/32.80) (2 x 16.00) = 63.54 amu

Copper.

**Solution #3:**

1) Determine the molar mass of M(OH)_{2}:

32.00 is to 0.3280 as x is to 1x = 97.56 g/mol

2) Subtract the weight of two hydroxides:

97.56 − 34.02 = 63.54 g/molCopper.

**Solution #4:**

1) Assume 100.0 g of the compound is present. Therefore, the mass of oxygen in the compound:

(100.0 g) (0.328) = 32.8 g

2) Moles of O in the compound:

32.8 g / 16.00 g/mole = 2.05 mole

3) Mass of H in the compound:

(2.05 mol ) (1.01 g/mol) = 2.07 g

4) Mass of metal in the 100 g of compound:

100.0 g − 32.8 g − 2.07 g = 65.13 g

5) From the formula:

moles of M = moles of O / 2 = 2.05 / 2 = 1.025 mol

6) Atomic weight of M:

65.13 g / 1.025 mol = 63.54 g/molCopper

**Example #9:** An oxide of an element with a valance of 6 contains 48% oxygen. What is the atomic weight of this element? Identify the element.

**Solution:**

An element, M, with a valence of +6 will form an oxide with the formula MO_{3}.Let us assume 100 g of the compound is present. Therefore, 48 g is oxygen and 52 g is M.

The moles of oxygen ---> 48 g / 16 g/mol = 3 mol

From the 1:3 molar ratio of M to O, we have this:

1 is to 3 as x is to 3 moles

x = 1 mole (This is how many moles of M are in the 100 g of MO

_{3}.)52 g / 1 mol = 52 g/mol <--- the atomic weight of M

The element is chromium.

**Example #10:** An oxybromide compound, KBrO_{x}, where x is unknown, is analyzed and found to contain 59.19% Br. What is the value of x?

**Solution:**

1) The three elements exist in the compound in a fixed ratio:

K : Br : O = 1 : 1 : x

2) Use the atomic weights to write an expression for the percent bromine:

59.19 = [(1) (79.9)] / { [(1) (39.1)] + [(1) (79.9)] + [(x) (16.0)] }59.19 = 79.9 / [119 + 16x]

3) We know x must be an integer, so let us try x = 1:

59.19 = 79.9 / [119 + 16]79.9 / [119 + 16] = 59.185

4) The formula for KBrO_{x} is:

KBrO

Problems #1 - 10 | Return to Mole Table of Contents |

Determine empirical formula when given mass data

Determine empirical formula when given percent composition data

Determine identity of an element from a binary formula and mass data