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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
103
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
moles of AlBr 3 ϭ grams AlBr 3 ϫ
moles of AlBr 3 ϭ 71.16 g AlBr 3 ϫ
ϭ
0.2668 mol AlBr 3
moles of H 2 O ϭ grams H 2 O ϫ
moles of H 2 O ϭ 28.84 g H 2 O ϫ
ϭ 1.601 mol H 2 O
Next, calculate the mole ratio of H 2 O to AlBr 3 .
mole ration
ϭ
ϭ 6.001
This ratio tells you that the hydrate is made up of six molecules of
water for each formula unit of AlBr 3 . Now you can write the molecular formula for the hydrate as AlBr 3 и 6H 2 O.
Practice Problems
35. Cerium(III) iodide (CeI 3 ) occurs as a hydrate with the composition 76.3% CeI 3 and 23.7% H 2 O. Calculate the formula for the
hydrate.
36. Cobalt(II) nitrate (Co(NO 3 ) 2 ) is used in ceramic glazes and produces a blue color when the ceramic is fired. This compound
exists as a hydrate whose composition is 37.1% water and
62.9% Co(NO 3 ) 2 by mass. Write the molecular formula for this
hydrate.
37. Lead(II) acetate (Pb(C 2 H 3 O 2 ) 2 ) exists as a crystalline hydrate
that is 14.25% water by mass. What is the molecular formula
for this hydrate?
38. A 17.44-g sample of a hydrate of zinc sulfate (ZnSO 4 ) is heated
strongly in a crucible to drive off all water of hydration. After
heating, the sample has a mass of 9.79 g. Calculate the molecular formula for this hydrate. (Hint: The mass lost represents
water. First determine the moles of water lost from the hydrate.)
1.601 mol H 2 O
ᎏᎏ
0.2668 mol AlBr 3
H 2 O
ᎏ
AlBr 3
1 mol H 2 O
ᎏᎏ
18.015 g H 2 O
1 mol H 2 O
ᎏᎏ
gram H 2 O
1 mol AlBr 3
ᎏᎏ
266.68 g AlBr 3
1 mol AlBr 3
ᎏᎏ
grams AlBr 3