The Mole and Avogadro's Number
Exercises
Some more practice problems can be found here (choose "Chapter 24 The Mole" when you get there.) You may have to cover part of your screen because the answers are right by the questions! (No peeking until you've first worked on the problem yourself!) These problems are from Don Voyce's chemistry site at the University of Hawai'i Kapi`olani Community College.
Exercises
1. How many moles of the substance are in the following amounts?
a) 2.50 g of lead
b) 5.00 g of KBr
c) 3.75 g of Ca3(PO4)2
d) 10.00 g of CH3CH2CH2CH2CH2CH3
e) 15.00 g of CuSO4 �?5 H2O
f) a cube of copper metal with an edge length of 1.00 in. The density of copper is 8.92 g/cm3.
2. How many atoms, molecules, or formula units of the substance are in the following amounts?
a) 3.50 mol of O2
b) 2.75 g of S8
c) 5.50 g of C12H22O11
d) 5.00 g of Al(NO3)3
e) 5.00 mL of ethanol, CH3CH2OH, density = 0.790 g/mL
f) a sphere of chromium metal, 0.343 mm in diameter. The density of chromium is 7.20 g/cm3.
3. How many moles of phosphate ions are in 1.00 lb of Ca3(PO4)2 ?
1 lb = 453.59237 g exactly
4. A student performed an experiment to determine the molecular weight of a gaseous compound. Using the ideal gas law PV = nRT, and knowing the pressure, temperature, and volume of the vapor, the student calculated the number of moles of gas, n, to be 0.0443 mol. The weight of the gas was 1.42 g. What is the molecular weight of this compound?
5. The stratosphere contains approximately 3 billion kilograms of ozone, O3. How many moles of ozone is this?
6. At the end of the 18th century, a kilogram was defined as the mass of exactly one cubic decimeter of water at the temperature where the density of water is at a maximum (now known to be 0.999972 g/cm3 at 3.98 °C and 1 atm pressure; see http://www.sizes.com/units/kilogram.htm). How many moles of water is this?
7. How many moles of water are in a snowflake weighing 5.0 X 10�? g ? How many of these snowflakes would it take to have 1.0 kg of ice?
8. What is the percent by mass of carbon in sucrose, C12H22O11 ?
9. What is the empirical formula of a compound that is 21.2% nitrogen, 6.1% hydrogen, 24.3% sulfur, and 48.4% oxygen by weight?
10. The mass of the earth is 5.98 X 1024 kg. How many moles of gold would have this mass? How many moles of aluminum would have this mass?
Answers
1. a) 0.0121 mol of Pb
b) 0.0420 mol of KBr
c) 0.0121 mol of Ca3(PO4)2
d) 0.116 mol of C6H14
e) 0.0601 mol of CuSO4 �?5 H2O
f) 2.30 mol of Cu
2. a) 2.11 X 1024 O2 molecules
b) 6.46 X 1021 S8 molecules
c) 9.68 X 1021 C12H22O11 molecules
d) 1.41 X 1022 Al(NO3)3 formula units
e) 5.16 X 1022 CH3CH2OH molecules
f) 4.76 X 1021 Cr atoms
3. 2.92 mol of PO43�?/SUP> ions
4. 32.1 g/mol
5. 6 X 1010 mol of O3
6. 55.5068 mol of H2O using a molecular weight of 18.0153 g/mol
7. 2.8 X 10�? mol of H2O, 2 X 107 or 20 million snowflakes
8. 42.1% carbon by mass
9. N2H8SO4 (ammonium sulfate)
10. 3.04 X 1025 moles of Au, 2.22 X 1026 mol of Al
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