I.
Percent Composition
H NT Chapter 11 part 2 and 3
a. Formula
b. Examples
i. Aluminum chloride
ii. Sulfuric acid (H2SO4)
iii. Calcium hydroxide
II.
Formula of a Hydrate
a. Hydrate
b. Anhydrate
A hydrate is an ionic compound that contains water molecules in its structure. To determine the formula of a hydrate
experimentally, we must calculate the mole: mole ratio of the water portion compared to the anhydrate portion. An
anhydrate is the substance that remains after the water is removed from a hydrate. When a hydrate is heated the
water molecules are driven off as steam, leaving behind the water-free anhydrate. The first step to finding the formula
for a hydrate is to record the mass of the hydrate. After heating the hydrate, the mass is determined for the anhydrate
that remains. The mass of the water that was present is calculated by finding the difference between the mass of the
hydrate and the mass of the anhydrate. The mass of the water and the mass of the anhydrate are each converted to
moles using their respective molar masses. From this a whole number ratio, X, can be determined. anhydrate  X
H2O
Prefix
MonoDiTriTetraPentaHexaHeptaOctaNonaDeca-
Molecules of H2O
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Formulas of Hydrates
Formula
(NH4)C2O4. H2O
CaCl2.2H2O
NaC2H3O2.3H2O
FePO4.4H2O
CuSO4.5H2O
CoCl2.6H2O
MgSO4.7H2O
Ba(OH)2.8H2O
Fe(NO3)3.9H2O
Na2CO3.10H2O
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Name
ammonium oxalate monohydrate
calcium chloride dihydrate
sodium acetate trihydrate
iron(III) phosphate tetrahydrate
copper(II) sulfate pentahydrate
cobalt(II) chloride hexahydrate
magnesium sulfate heptahydrate
barium hydroxide octahydrate
iron(III) nitrate nonahydrate
sodium carbonate decahydrate
c. Steps to find formula
1.
2.
3.
4.
d. Examples
i. A calcium chloride hydrate has a mass of 4.72 g. After heating for several minutes
the mass of the anhydrate is found to be 3.56 g. Use this information to determine
the formula for the hydrate. Name the hydrate.
ii. A 15.67 g sample of a hydrate of magnesium carbonate was heated, without
decomposing the carbonate, to drive off the water. The mass of the anhydrate was
found to be 7.58 g. What is the formula and name the hydrate.
iii. What is the formula and name for a hydrate which consists of 76.9% CaSO3 and
23.1% H2O?
iv. What is the formula and name for a hydrate which consists of 51.77% NiSO4 and
48.23% H2O?
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III.
Empirical Formula
a. Steps to determine
i.
ii.
iii.
iv.
b. Examples
i. Determine the empirical formula for a compound with the percent composition of
40.00% carbon, 6.72% hydrogen, and 53.28% oxygen.
ii. Determine the empirical formula for methyl acetate, which has the following
chemical analysis: 48.64% carbon, 8.16% hydrogen and 43.20% oxygen.
iii. A 50.0 g sample of a compound contains 18.25 g Na, 19.05 g O and 12.70 g S.
Determine the empirical formula.
iv. A 50.0 gram sample contains 2.52 grams of Be, 5.01 g Al, 15.68 g Si, and 26.97 g O.
Determine the empirical formula.
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IV.
Molecular Formula
a. Steps to determine
i.
ii.
iii.
iv.
b. Examples
i. A compound composed of 40.68% carbon, 5.08% hydrogen and 54.24% oxygen has
a molar mass of 118.1 g/mol. Determine the empirical and molecular formula.
ii. A compound has a molar mass of 462.8 g/mol and contains 77.87% C, 11.76% H,
and 10.37% O. Determine the empirical and molecular formulas.
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