# 2.3.3: Formula Mass, Percent Composition, and the Mole (Problems)

PROBLEM $$\PageIndex{1}$$

What is the total mass (amu) of carbon in each of the following molecules?

(a) CH4
(b) CHCl3
(c) C12H10O6
(d) CH3CH2CH2CH2CH3

12.01 amu

12.01 amu

144.12 amu

60.05 amu

PROBLEM $$\PageIndex{2}$$

What is the total mass of hydrogen in each of the molecules?

(a) CH4
(b) CHCl3
(c) C12H10O6
(d) CH3CH2CH2CH2CH3

4.032 amu

1.008 amu

10.08 amu

12.096 amu

PROBLEM $$\PageIndex{3}$$

Calculate the molecular or formula mass of each of the following:

(a) P4
(b) H2O
(c) Ca(NO3)2
(d) CH3CO2H (acetic acid)
(e) C12H22O11 (sucrose, cane sugar).

123.896 amu

18.015 amu

164.086 amu

60.052 amu

342.297 amu

PROBLEM $$\PageIndex{4}$$

Determine the molecular mass of the following compounds:

(a) (b) (c) (d) 98.906 amu

26.018 amu

185.818 amu

98.072 amu

PROBLEM $$\PageIndex{5}$$

Determine the molecular mass of the following compounds:

(a) (b) (c) (d) 56.107 amu

54.091 amu

199.9976 amu

97.9950 amu

PROBLEM $$\PageIndex{6}$$

Compare 1 mole of H2, 1 mole of O2, and 1 mole of F2.

(a) Which has the largest number of molecules? Explain why.

(b) Which has the greatest mass? Explain why.

1 mole is always 6.022 x 1023 molecules. They have the same number of molecules.

F2; it has the highest molar mass.

PROBLEM $$\PageIndex{7}$$

Which contains the greatest mass of oxygen: 0.75 mol of ethanol (C2H5OH), 0.60 mol of formic acid (HCO2H), or 1.0 mol of water (H2O)? Explain why.

Formic acid. Its formula has twice as many oxygen atoms as the other two compounds (one each). Therefore, 0.60 mol of formic acid would be equivalent to 1.20 mol of a compound containing a single oxygen atom.

PROBLEM $$\PageIndex{8}$$

Calculate the molar mass of each of the following:

(a) S8
(b) C5H12
(c) Sc2(SO4)3
(d) CH3COCH3 (acetone)
(e) C6H12O6 (glucose)

256.528 g/mol

72.150 g/mol

378.103 g/mol

58.080 g/mol

180.158 g/mol

PROBLEM $$\PageIndex{9}$$

Calculate the molar mass of each of the following:

(a) the anesthetic halothane, C2HBrClF3
(b) the herbicide paraquat, C12H14N2Cl2
(c) caffeine, C8H10N4O2
(d) urea, CO(NH2)2
(e) a typical soap, C17H35CO2Na

197.382 g/mol

257.163 g/mol

194.193 g/mol

60.056 g/mol

306.464 g/mol

PROBLEM $$\PageIndex{10}$$

Determine the number of moles of compound and the number of moles of each type of atom in each of the following:

(a) 25.0 g of propylene, C3H6
(b) 3.06 × 10−3 g of the amino acid glycine, C2H5NO2
(c) 25 lb of the herbicide Treflan, C13H16N2O4F (1 lb = 454 g)
(d) 0.125 kg of the insecticide Paris Green, Cu4(AsO3)2(CH3CO2)2
(e) 325 mg of aspirin, C6H4(CO2H)(CO2CH3)

0.595 mol C3H6

1.78 mol C

3.57 mol H

4.08 × 10-5 mol C2H5NO2

8.16 × 10-5 mol C

2.04 × 10-4 mol H

4.08 × 10-5 mol N

8.16 × 10-5 mol O

40.106 mol C13H16N2O4F

521.37 mol C

641.70 mol H

80.212 mol N

160.42 mol O

40.106 mol F

0.202 mol Cu4(AsO3)2(CH3CO2)2

0.808 mol Cu

0.404 mol As

2.02 mol O

0.808 mol C

1.212 mol H

0.00181 mol C6H4(CO2H)(CO2CH3)

0.01625 mol C

0.0144 mol H

0.0072 mol O

PROBLEM $$\PageIndex{11}$$

Determine the mass of each of the following:

(a) 0.0146 mol KOH
(b) 10.2 mol ethane, C2H6
(c) 1.6 × 10−3 mol Na2 SO4
(d) 6.854 × 103 mol glucose, C6 H12 O6
(e) 2.86 mol Co(NH3)6Cl3

0.819 g

307 g

0.23 g

1.235 × 106 g (1235 kg)

765 g

PROBLEM $$\PageIndex{12}$$

Determine the number of moles of the compound and determine the number of moles of each type of atom in each of the following:

(a) 2.12 g of potassium bromide, KBr
(b) 0.1488 g of phosphoric acid, H3PO4
(c) 23 kg of calcium carbonate, CaCO3
(d) 78.452 g of aluminum sulfate, Al2(SO4)3
(e) 0.1250 mg of caffeine, C8H10N4O2

0.0178 mol KBr

0.0178 mol K

0.0178 mol Br

0.00152 mol H3PO4

0.00455 mol H

0.00152 mol P

0.00607 mol O

230 mol CaCO3

230 mol Ca

230 mol C

690 mol O

0.229 mol Al2(SO4)3

0.459 mol Al

0.688 mol S

2.75 mol O

6.44 × 10-7 mol C8H10N4O2

5.15 × 10-6 mol C

6.44 × 10-6 mol H

2.58 × 10-6 mol N

1.29 × 10-6 mol O

PROBLEM $$\PageIndex{13}$$

The approximate minimum daily dietary requirement of the amino acid leucine, C6H13NO2, is 1.1 g. What is this requirement in moles?

0.0084 mol C6H13NO2

PROBLEM $$\PageIndex{14}$$

Determine the mass in grams of each of the following:

(a) 0.600 mol of oxygen atoms
(b) 0.600 mol of oxygen molecules, O2
(c) 0.600 mol of ozone molecules, O3

9.60 g

19.2 g

28.2 g

PROBLEM $$\PageIndex{15}$$

A 55-kg woman has 7.5 × 10−3 mol of hemoglobin (molar mass = 64,456 g/mol) in her blood. How many hemoglobin molecules is this? What is this quantity in grams?

4.52 × 1021 molecules of hemoglobin

483.42 g of hemoglobin

PROBLEM $$\PageIndex{16}$$

One 55-gram serving of a particular cereal supplies 270 mg of sodium, 11% of the recommended daily allowance. How many moles and atoms of sodium are in the recommended daily allowance?

0.012 mol Na; 7.06 × 1021 atoms Na

PROBLEM $$\PageIndex{17}$$

A tube of toothpaste contains 0.76 g of sodium monofluorophosphate (Na2PO3F) in 100 mL.

1. What mass of fluorine atoms in mg was present?
2. How many fluorine atoms were present?

100.27 mg F

3.18 × 1021 atoms F

PROBLEM $$\PageIndex{18}$$

Which of the following represents the least number of molecules?

1. 20.0 g of H2O (18.02 g/mol)
2. 77.0 g of CH4 (16.06 g/mol)
3. 68.0 g of CaH2 (42.09 g/mol)
4. 100.0 g of N2O (44.02 g/mol)
5. 84.0 g of HF (20.01 g/mol)

20.0 g of H2O represents the smallest number of moles, meaning the least number of molecules present. Since 1 mole = 6.022 × 1023 molecules (or atoms) regardless of identity, the least number of moles will equal the least number of molecules.

PROBLEM $$\PageIndex{19}$$

Calculate the following:

(a) the percent composition of ammonia, NH3
(b) the percent composition of photographic “hypo,” Na2S2O3
(c) the percent of calcium ion in Ca3(PO4)2

% N = 82.24%

% H = 17.76%

% Na = 29.08%

% S = 40.56%

% O = 30.36%

% Ca2+ = 38.76%

PROBLEM $$\PageIndex{20}$$

A compound of carbon and hydrogen contains 92.3% C and has a molar mass of 78.1 g/mol. What is its molecular formula?

C6H6

PROBLEM $$\PageIndex{21}$$

Dichloroethane, a compound that is often used for dry cleaning, contains carbon, hydrogen, and chlorine. It has a molar mass of 99 g/mol. Analysis of a sample shows that it contains 24.3% carbon and 4.1% hydrogen. What is its molecular formula?

C2H4Cl2