4.2: Formula Mass, Percent Composition, and the Mole (Problems)
- Page ID
- 98702
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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
- Answer a
-
12.01 amu
- Answer b
-
12.01 amu
- Answer c
-
144.12 amu
- Answer d
-
60.05 amu
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PROBLEM \(\PageIndex{2}\)
What is the total mass of hydrogen in each of the molecules?
(a) CH4
(b) CHCl3
(c) C12H10O6
(d) CH3CH2CH2CH2CH3
- Answer a
-
4.032 amu
- Answer b
-
1.008 amu
- Answer c
-
10.08 amu
- Answer d
-
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).
- Answer a
-
123.896 amu
- Answer b
-
18.015 amu
- Answer c
-
164.086 amu
- Answer d
-
60.052 amu
- Answer e
-
342.297 amu
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PROBLEM \(\PageIndex{4}\)
Determine the molecular mass of the following compounds:
(a)
(b)
(c)
(d)
- Answer a
-
98.906 amu
- Answer b
-
26.018 amu
- Answer c
-
185.818 amu
- Answer d
-
98.072 amu
PROBLEM \(\PageIndex{5}\)
Determine the molecular mass of the following compounds:
(a)
(b)
(c)
(d)
- Answer a
-
56.107 amu
- Answer b
-
54.091 amu
- Answer c
-
199.9976 amu
- Answer d
-
97.9950 amu
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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.
- Answer a
-
1 mole is always 6.022 x 1023 molecules. They have the same number of molecules.
- Answer b
-
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.
- Answer
-
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)
- Answer a
-
256.528 g/mol
- Answer b
-
72.150 g/mol
- Answer c
-
378.103 g/mol
- Answer d
-
58.080 g/mol
- Answer e
-
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
- Answer a
-
197.382 g/mol
- Answer b
-
257.163 g/mol
- Answer c
-
194.193 g/mol
- Answer d
-
60.056 g/mol
- Answer e
-
306.464 g/mol
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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
(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)
- Answer a
-
0.595 mol C3H6
1.78 mol C
3.57 mol H
- Answer b
-
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
- Answer c
-
40.106 mol C13H16N2O4F
521.37 mol C
641.70 mol H
80.212 mol N
160.42 mol O
40.106 mol F
- Answer d
-
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
- Answer e
-
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
(d) 6.854
(e) 2.86 mol Co(NH3)6Cl3
- Answer a
-
0.819 g
- Answer b
-
307 g
- Answer c
-
0.23 g
- Answer d
-
1.235 × 106 g (1235 kg)
- Answer e
-
765 g
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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
- Answer a
-
0.0178 mol KBr
0.0178 mol K
0.0178 mol Br
- Answer b
-
0.00152 mol H3PO4
0.00455 mol H
0.00152 mol P
0.00607 mol O
- Answer c
-
230 mol CaCO3
230 mol Ca
230 mol C
690 mol O
- Answer d
-
0.229 mol Al2(SO4)3
0.459 mol Al
0.688 mol S
2.75 mol O
- Answer e
-
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?
- Answer
-
0.0084 mol C6H13NO2
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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
- Answer a
-
9.60 g
- Answer b
-
19.2 g
- Answer c
-
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?
- Answer
-
4.52 × 1021 molecules of hemoglobin
483.42 g of hemoglobin
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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?
- Answer
-
0.107 mol Na; 6.45 × 1022 atoms Na
PROBLEM \(\PageIndex{17}\)
A tube of toothpaste contains 0.76 g of sodium monofluorophosphate (Na2PO3F) in 100 mL.
- What mass of fluorine atoms in mg was present?
- How many fluorine atoms were present?
- Answer a
-
100.27 mg F
- Answer b
-
3.18 × 1021 atoms F
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PROBLEM \(\PageIndex{18}\)
Which of the following represents the least number of molecules?
- 20.0 g of H2O (18.02 g/mol)
- 77.0 g of CH4 (16.06 g/mol)
- 68.0 g of CaH2 (42.09 g/mol)
- 100.0 g of N2O (44.02 g/mol)
- 84.0 g of HF (20.01 g/mol)
- Answer
-
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
- Answer a
-
% N = 82.24%
% H = 17.76%
- Answer b
-
% Na = 29.08%
% S = 40.56%
% O = 30.36%
- Answer c
-
% Ca2+ = 38.76%
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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?
- Answer
-
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?
- Answer
-
C2H4Cl2
- Click here to see a video of the solution
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In the second edition, this problem was moved. The number in the title of the video may be incorrect, but the solution is correct.
Contributors
Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).
- Adelaide Clark, Oregon Institute of Technology