Download Chemistry Practice Problems

Chemistry Practice Problems
Compounds: Chemical formulas from experimental data
Solutions: 1. What is the empirical formula of the compound containing 39.8% Cu, 20.1% S, and
40.1% O by mass?
1.1. Assume a 100 g sample.
39.8 g Cu, 20.1 g S, and 40.1 g O
1.2. Convert grams to moles.
⎛ 1 mol Cu ⎞
39.8 g Cu ⎜
⎟ = 0.626 mol Cu
⎝ 63.55 g ⎠
⎛ 1 mol S ⎞
20.1 g S ⎜
⎟ = 0.627 mol S
32.06
g
⎝
⎠
⎛ 1 mol O ⎞
40.1 g O ⎜
⎟ = 2.51 mol O
⎝ 16.00 g ⎠
1.3. Divide by the lowest number of moles.
0.626 mol Cu
= 1.00
0.626
0.627 mol S
= 1.01
0.626
2.51 mol O
= 4.01
0.626
1.4. Write the formula using the results of the previous step as subscripts (values of
1 can be omitted) to find the empirical formula.
CuSO4
1.4.1. Since all of the subscripts are whole numbers, it is not necessary to
multiply through by an integer.
Chemistry Practice Problems
2. What is the empirical formula of benzene, a hydrocarbon, which contains 92.3%
carbon by mass?
2.1. A hydrocarbon contains carbon and hydrogen. The mass percent of carbon is
given which means the remaining amount must be hydrogen. Therefore, the
mass percent of hydrogen is 7.7%.
2.2. Assume a 100 g sample.
92.3 g C and 7.7 g H
2.3. Convert grams to moles.
⎛ 1 mol C ⎞
92.3 g C ⎜
⎟ = 7.69 mol C
12.01
g
⎝
⎠
⎛ 1 mol H ⎞
7.7 g H ⎜
⎟ = 7.6 mol H
⎝ 1.008 g ⎠
2.4. Divide by the lowest number of moles.
7.69 mol C
= 1.01
7.6
7.6 mol H
= 1.00
7.6
2.5. Write the formula using the results of the previous step as subscripts (values of
1 can be omitted).
CH
2.6. Since all of the subscripts are whole numbers, it is not necessary to multiply
through by an integer.
Chemistry Practice Problems
3. What is the chemical formula of a compound that contains 69.7% C, 11.7% H, and
18.6% O by mass and has an approximate molar mass of 172 g/mol?
3.1. For this problem, first find the empirical formula and then use the approximate
molar mass to find the chemical formula.
3.2. Assume a 100 g sample.
69.7 g C, 11.7 g H, and 18.6 g O
3.3. Convert grams to moles.
⎛ 1 mol C ⎞
69.7 g C ⎜
⎟ = 5.80 mol C
12.01
g
⎝
⎠
⎛ 1 mol H ⎞
11.7 g H ⎜
⎟ = 11.6 mol H
⎝ 1.008 g ⎠
⎛ 1 mol O ⎞
18.6 g O ⎜
⎟ = 1.16 mol O
16.00
g
⎝
⎠
3.4. Divide by the lowest number of moles.
5.80 mol C
= 5.00
1.16
11.6 mol H
= 10.0
1.16
1.16 mol O
= 1.00
1.16
3.5. Write the formula using the results of the previous step as subscripts (values of
1 can be omitted).
C5H10O
3.5.1. Since all of the subscripts are whole numbers, it is not necessary to
multiply through by an integer to find the empirical formula.
Chemistry Practice Problems
3.5.2. To find the chemical formula, determine the empirical formula mass from
the previously determined empirical formula.
C5H10O
5(12.01) + 10(1.008) + 1(16.00) = 86.13 g/mol
C 12.01 g/mol
H 1.008 g/mol
O 16.00 g/mol
3.6. Determine the ratio between the given molar mass and the empirical formula
mass.
172
=2
86.13
3.7. The ratio is 2 so the chemical formula will be the empirical formula multiplied
by 2.
C10H20O2