Chemistry 400
LECTURE PROBLEMS SET #1:
ISOTOPES, EMPIRICAL FORMULAS, AND MOLES
1. Copper consists of only two isotopes with masses and abundances of 62.95 (70.5%) and
64.96 (??%). Calculate the atomic mass of Cu to the correct number of significant figures
from the data. ANS: see periodic chart.
2. Magnesium consists of three isotopes with masses and abundances of 23.96 (78.6%),
24.98 (10.1%) and 25.98 (??%). Calculate the average atomic mass in amu of
magnesium. ANS: You know where!
3. Bromine consists of two isotopes with masses of 78.92 and 80.82 amu each. Estimate the
abundance of each to the correct number of significant digits. Check by plugging the
numbers back into the equation.
4. How many molecules of oxygen are there in 10.8 g oxygen? How many individual atoms
are present?
ANS: 0.675 moles of atoms O and 2.03x1023 molecules O2.
5. Some quickies
a) How many moles are in 0.4 g of helium gas?
b) How many moles are in 280 g of nitrogen gas?
c) How many g of oxygen gas are in 0.10 moles?
d) How many atoms are in 0.32 g of sulfur?
e) How many individual formula units of NaCl are in 116g of sodium chloride?
6. Calculate the molar mass of Ni(II)bromide trihydrate. NiBr2•3H2O. ANS: 272.5g/mol
NiBr2•3H2O
7. How many grams of calcium are in 3.1 g of calcium phosphate? ANS: 1.2 g
8. How many atoms of Cl in 12.5g of CaCl2? The molar mass of CaCl2 is 110.98 g/mol.
ANS: 1.36*1023 atoms Cl
9. How many moles of acetone (C3H6O) in 19.3 ml of acetone (D=0.821 g/ml)?
ANS: 0.273moles acetone
10. What is mass percent of Xe and F in XeF4? ANS: 63.34% Xe; 36.66% F
11. Hemoglobin contains 0.342% Fe by mass. Each molecule of hemoglobin contains 4 Fe
atoms. What is the molar mass of hemoglobin? ANS: 6.53 x 104 g/mole heme
12. The empirical formula for Vitamin C is C3H4O3. Vitamin C has molar mass of 176g/mol.
What is the molecular formula for vitamin C? ANS: C6H8O6
13. Iosoamyl formate, a substance used in artificial fruit syrups, is analyzed for %
composition with the following results:
62.0%C
10.4% H
27.6% O
Determine the empirical and molecular formulas for isoamyl formate.
ANS: empirical formula = C3H6O; molecular formula = C6H12O2
14. An analysis of nicotine, a poisonous compound found in tobacco leaves, shows that it is
74.03% carbon, 8.70% hydrogen, and the remainder is nitrogen. By a separate analysis,
its molar mass is found to be 162 g/mol. What are the empirical and molecular formulas
of nicotine? ANS: empirical formula = C5H7N; molecular formula = C10H14N2
15. Nickel (II) Bromide forms a stable hydrate with an unknown number of waters of
hydration NiBr2•xH2O. After heating a 4.314 g sample, the residue weighs 3.459g. What
is the formula of the nickel bromide hydrate? ANS: NiBr2•3H2O
16. Acetylene is a colorless gas, a hydrocarbon, which is used as a fuel in welding torches. It
is 92.25% carbon and the remainder is hydrogen. Its molecular mass is 26.04 g/mol.
Calculate the empirical and molecular formulas. ANS: C1H1; C2H2