CHEMICAL QUANTITIES ANALYZING COMPOUNDS
Chapter 10
PERCENT COMPOSITION
• You may recall that Proust stated that samples of the
same compound will contain the same ration of
elements by mass
• This is the basis for identifying a compound based on
its percent composition • A statement of the relative mass of each
component or element in the compound.
CALCULATING PERCENT COMPOSITION
• Remember that percentage is calculated by dividing the
part by the whole
• Determine the molar mass of the compound (whole)
• Al2(SO4)3
• Molar mass = 342 g/mol
• Find the molar mass of the component element – don’t
forget to multiply by the number of atoms of that
element in the compound (part)
• Al - mass of 27
• 2 atoms of Al in compound = 54
• Divide the mass of the element by the mass of the
compound (part over whole)
• Multiply by 100%
• 54/342 x 100% = 15.8%
% COMPOSITION FROM EXPERIMENTATION
• It was determined that an unkown sample of a solid ore
contained 106g of Ca and 188g of Cl. What is the percent of
calcium in the ore?
• 106 g of Ca in the sample
• Total sample is 106 + 188 = 294
• %Ca = 106/294 x 100% = 36%
• If I have 23.6g of a gas mixture that contains 12g of CO2, what %
of the mixture is carbon?
• Molar mass of CO2 is 44.01g
• C is 12.01g/44.01g CO2 = 27%
• Total carbon is 27% of the 12 g of CO2 = 3.27g of carbon
• 3.27g C / 23.6g of mixture x 100 = 13.86% C in the mixture
HYDRATED SALTS
• When ionic satls are exposed to moist air, the polar
water molecules will be attracted to the ions in the
crystalline structure.
• When using these hydrated salts it is necessary to
account for the salt as well as the water in the mass,
so the number of water molecules are indicated in
the formula
• Example: CuSO4  6 H2O represents copper (II)
sulfate hexahydrate
• This tells us that there are 6 water molecules in
the sample for every formula unit of CuSO4
% COMPOSITION OF A HYDRATE
• What
is the percent of nickel in nickel (III) nitrate heptahydrate?
• Write the formula for the hydrate
• Ni(NO3)3  7H2O
• Determine the molar mass of the hydrate
• Ni = 58.69 g/mol
• N = 14.01 g/mol X 3 = 42.03
• O = 16.0 g/mol X 9 = 144.0
• H = 1.01 g/mol X 14 = 14.14
• O = 16.0 g/mol x 7 = 112.0
• = 312.17 g/mol
• Divide the molar mass of the nickel by the molar mass of the
hydrate
• 58.69 g/mol / 312.17 g/mol X 100%
• = 18.80%
% COMPOSITION AS AN IDENTIFIER
• If you can experimentally determine the masses or percent
of each element in a sample, you can work backwards
from the composition to determine the chemical formula.
• When we determine the formula we find the empirical
formula
• The lowest ration of atoms in a molecule
• The simplest form of the formula
CALCULATE EMPIRICAL FORMULA
• A sample is 59.53% C, 5.38%H, 10.68%N, and 24.40%O what is its empirical formula
• Convert the % to a mass
• Assume sample had a mass of 100g, then the percent to grams is a direct conversion
• 54.88g C; 5.38g H, 10.68g N, 24.4g O
• Convert mass of each element to moles
• 54.88g C/ 1 mol C = 4.570 mol C
1 / 12.01g C
• 5.38g H/ 1 mol H = 5.327 mol H
1 / 1.01g H
• 10.68g N/ 1 mol N = .7623 mol N
1 / 14.01g N
• 24.4gO / 1 mol O = 1.525 mol O
1 / 16.0g O
• Determine the ratio
• Since formulas are whole number ratios, we need the smallest number of moles to be 1 – so
we will divide all the moles by the smallest one.
• O = 1.525/.7623 = 2.00
• N = .7623/.7623 = 1.00
• H = 5.327/.7623 = 6.99
• C = 4.961/.7623 = 6.00
• C6H7NO2
EMPIRICAL TO MOLECULAR FORMULAS
• Empirical is lowest ratio.
• Molecular is actual molecule.
• Related by Molar mass.
• Ratio of empirical to molar mass will tell you the
molecular formula.
• Must be a whole number
EXAMPLE
• The empirical formula of carbohydrates is CH2O. What
would be the molecular formula of a carbohydrate with a
molecular mass of 150 g/mol?
• The molar mass of CH2O is 30 g/mol
• The ratio between the molar masses and the ratio
between the formulas will be the same, so divide the
molecular mass by 30 to determine the multiplier
• 150 g/mol/ 30 g/mol = 5
• Multiply the empirical formula times 5
• C5H10O5
EXAMPLE
• A compound is made of only sulfur and oxygen. It is
69.6% S by mass. Its molar mass is 184 g/mol. What is
its formula?