HMSA Chemistry
Ms. Ye
Name __________________________________
Date _______________________Block______
Review of the Mole:
Human Bones: The chemical equation for the reaction that forms calcium phosphate, the main
ingredient in bones, is this:
Calculate the molar mass of each substance and fill in the table. SHOW WORK!
Imagine that this reaction is repeated in the laboratory using different amounts of reactants. Complete
the table. SHOW WORK!
4.00 mol
*How many molecules of Ca 3(PO4)2 are in 310 grams of Ca 3(PO4)2?
HMSA Chemistry
Ms. Ye
Name __________________________________
Date _______________________Block______
Molecular Formula vs. Empirical Formula
 Molecular Formula: represents the ________________________________________
o Ex: C11H22O11, H2O2
 Empirical Formula: represents the _________________________________________
o Ex: CH2O, HO
To convert a molecular formula to an empirical formula:

divide the subscripts by the greatest common factor to get your new subscript
o most of the time, this can be accomplished by dividing the subscripts by the smallest
subscript
o note: when you divide the subscripts, your answer has to be a WHOLE NUMBER. You
cannot have decimals as subscripts.
Ex: C2H4
Greatest common factor: 2
Divide both subscripts by 2: 𝐶 2 𝐻4 → 𝑪𝑯𝟐
2
2
Practice: Determine the empirical formulas for the compounds. If it helps you to draw out the
individual atoms, please do so.
*Note: if the formula cannot be simplified further, the empirical formula is the same as the molecular
formula
1. Al2Br6
4. C6H8O6
2. Na2S2O4
5. C2H6
3. N2O4
6. HgI2
Empirical Formula (E.F.)
CH2
CaCl2
CF3
“Mass” of E.F.
Molar Mass of M.F.
84.18 g/mol
110.98 g/mol
138.02 g/mol
Molecular Formula (M.F.)
HMSA Chemistry
Ms. Ye
Name __________________________________
Date _______________________Block______
Percent Composition: Identifies the elements present in a compound as a mass percent of the total
compound mass
How to calculate percent composition of an element in a compound:
1. Identify or determine the formula mass
of the compound (in grams)
Ex: K2CO3
 K: 2 x 39.1 grams = 78.2 grams
 C: 1 x 12.0 grams= 12.0 grams
 O: 3 x 16.0 grams = 48.0 grams
 Total Mass= 138.2 grams
2. Divide the total mass of each element by the
formula mass. Multiply by 100% to convert to
a percentage.
78.2 𝑔𝑟𝑎𝑚𝑠

%K = 138 .2 𝑔𝑟𝑎𝑚𝑠 × 100% = 𝟓𝟔. 𝟓𝟖%

%C =138.2 𝑔𝑟𝑎𝑚𝑠 × 100% = 𝟖. 𝟔𝟖%

%O =
12.0 𝑔𝑟𝑎𝑚𝑠
48 .0 𝑔𝑟𝑎𝑚𝑠
138 .2 𝑔𝑟𝑎𝑚𝑠
× 100% = 𝟑𝟒. 𝟕𝟑%
Practice: determine the percent compositions of all of the elements in the following compounds:
Molecular Formula Gram Formula Mass
Percent Composition (%)
%H=
H2O2
%O=
%N=
NH3
%H=
%C=
C3H6O3
%H=
%O=