Chem 100 Exam 4 Practice Problems
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What is the mass relation of gallium to boron?
Given 41.3 grams of boron, how much gallium would this be equivalent to?
What is the mass of 39.0 moles of boron?
What is the molar mass of acetaminophen, which has a formula of C8H9O2N?
Given 8.75 * 10-5 mol of Na2CO3, calculate the following:
a) number of moles of O atoms
b) number of grams of C
c) number of Na ions
d) mass of Na2CO3
What is the percent composition of each of the elements in Na2CO3?
5. You discover new elements: Chem (Ch), Is (Is), and Fun (Fu). They combine to give the
empirical formula Ch3Is2Fu5. The molar mass of this empirical formula is 94.62 g/mol. The
molar mass of the molecular formula is 283.4 g/mol. What is the molecular formula?
6. Given: 2 C4H10 + 13 O2  8 CO2 + 10 H2O, calculate the following:
a) mole ratio between O2 and H2O
b) mole ratio between CO2 and H2O
c) how many moles of CO2 would be produced from 0.115 mol O2
d) mass of CO2 produced from 8.67 mol C4H10
e) mass of O2 which would react with 155 g C4H10
f) number of molecules produced from 6.44 * 10-6 g O2
7. Given: 4 NH3 + 5 O2  4 NO + 6 H2O
a) What is the mass of H2O produced from a mixture of 100 g NH3 and 200 g O2?
b) How much of the other reactant is in excess?
c) If you obtained 64.5 g H2O experimentally, what would the percent yield be?
8. A compound has a composition of 18.7% B, 20.7% C, 5.15% H, and 55.4% O. Its molar
mass is about 115 g/mol. What is the molecular formula?
Chem 100 Exam 4 Practice Problem Answers
1. Molar mass of gallium = 69.7 a.m.u.
Molar mass of boron = 10.8 a.m.u.
Mass relation of Ga to B = 69.7/10.8 or 6.45/1 (or any other equivalent ratio)
2. given 41.3 g B:
41.3 g B * (6.45 g Ga / 1 g B) = 266.4 g B
3. 39.0 moles B * 10.8 g B / 1 mol B = 421.2 g B
(note: 10.8 g is the atomic mass of B from the periodic table)
4. C8H9O2N: C: 8 * 12.0 = 96.0
H: 9 * 1.0 = 9.0
96.0 + 9.0 + 32.0 + 14.0 = 151.0 g/mol
O: 2 * 16.0 = 32.0 N: 1 * 14.0 = 14.0