Chem 1010 Tutorials
Tutorial 3B – Formula and Molecular Mass,
Molar Mass and Percent Composition
Fall 2013
1.
Calculate the molecular or formula masses of the following:
a) O2
b) potassium chloride
c) Na2SO4
d) Ca3(PO4)2
e) CuSO4 . 5 H2O
f) phosphoric acid
g) Na2[Pt(CN)4] • 3 H2O
2.
How many moles of oxygen atoms are combined with 4.20 moles of chlorine atoms in Cl2O7?
3.
a) How many moles of silicon are in 4.60 g of Si?
b) How many grams of Na2CO3 must be measured to obtain 0.125 mol of Na2CO3?
4.
a) How many moles of water are present in 20.0 g of water?
b) How many moles of hydrogen atoms are in this amount?
c) How many molecules of water are in this amount? (Hint: use Avogadro’s number.)
5.
a) Calculate the number of moles of ions in 3.3 moles of MgBr2.
b) The number of ions.
c) How many molecules are in 34.0 mL of ethanol, CH3CH2OH, (density = 0.789g/mL)?
6.
a) How many grams of NaCl contain 3.011 × 1023 Na+ ions?
b) What is the mass in grams of 1.00 x 107 H2O molecules?
c) How many atoms are in a sample of uranium with a mass of 1.00 μg?
7.
Without doing detailed calculations, which of the following samples contains the greatest
number of atoms? Explain your choice.
a) 1.0 mol Cl2
b) 50.0 g of Na
c) 17.0mL H2O (density = 1.0g/mL)
d) 1.2 x 1024 Mg atoms
8.
Without doing detailed calculations, state which of the following samples contains the greatest
mass? Explain your choice.
a) 0.80mol Fe
b) 1.1 x 1024 Mg atoms
c) 50.0 mL H2O (density = 1.0 g/mL)
d) 41.0g Al2(SO4)3
9.
25.0 g of methane react with oxygen to form 56.1 g of carbon dioxide and 68.6 g of water. How
many grams of oxygen must have reacted?
a) 124.7g
b) 99.7 g
c) 56.1 g
d) 149.7 g
e) 25.0 g
10.
A sample of pure carbon of mass 1.48 g was burned in an excess of air. The only product was
5.42 g of carbon dioxide. In a second experiment, an unknown mass of carbon was burned and
gave 11.62 g of carbon dioxide. What was the mass of carbon burned in the second
experiment?
a) 0.866 g
b) 3.17 g
c) 3.64 g
d) 11.6 g
e) 42.6 g
11.
Which of the following statements is FALSE?
a) one mole of H2O consists of 2 moles of H atoms and 1 mole of O atoms
b) one mole of H atoms has a mass of 1.0079 grams
c) one mole of C atoms has a mass of 12.011 grams
d) one mole of H atoms has a mass of 1.0079 amu
12.
The element lead occurs combined with various other elements in nature. One of the lead
containing minerals is matlockite, which is composed of lead (II) chloride (PbCl2(s)). Calculate the
mass of lead (Pb) that could be extracted from 105 kg of matlockite?
13.
Calculate the percent composition of the following compounds:
a) Fumaric acid, C4H4O4
b) Cocaine, C17H21NO4
14.
What is the mass percent of H2O in magnesium chloride hexahydrate?
ANSWERS:
1. a) 31.9988 amu
e) 249.685 amu
b) 74.551 amu
f) 97.9951 amu
c) 142.042 amu
g) 399.173 amu
d) 310.177 amu
2. 14.7 mol of oxygen atoms
3. a) 0.164 mol of Si
b) 13.2 g of Na2CO3
4. a) 1.11 mol of H2O
b) 2.22 mol of H atoms
5. a) 9.9 mol of ions
b) 6.0 x 10 ions
6. a) 29.22 g of NaCl
b) 2.99 x 10
7. C
8. C
9. B
10. B
11. D
12. 78.2 kg
24
-16
13. a) C - 41.3905%; H - 3.4734%; O - 55.1361%
14. 53.1677% H2O
23
c) 6.68 x 10 molecules of H2O
23
c) 3.51 x 10 molecules of ethanol
g of H2O
15
c) 2.53 x 10 atoms of U
b) C - 67.30855%; H - 6.97734%; N - 4.61731%; O - 21.0968%