Honors Chemistry
Name: ______________________________________ Date: ________________ Mods: __________
Formula Weight and Percent Composition

Example calculations of formula weight (this term is used for ionic compounds):
1) CaCl2-

2) Mg3(PO4)2-
Example calculation of molecular weight (this term is used for molecular compounds):
3) C2H6 (ethane)-

Extra Formula Weight Practice:
4)
5)
Mg(OH)2
F2
Name:
Name:
FW:
FW:
6)
7)
CH4
Co2(Cr2O7)3
Name:
Name:
FW:
FW:
Percent Composition: In-Class Examples
The percent by mass of an element in a compound is the number of grams of the element divided by
the number of grams of the compound, multiplied by 100. Think “part” over “whole”.
% by mass A=
(atomic mass of element A) (# of atoms of element A)
x 100
Formula weight of whole compound
1)
Magnesium nitrate
% Mg: _____________
2)
Formula: _______________________
% N: _____________
Tetrasulfur decaoxide
Formula: _______________________
% S: _____________
3)
Aluminum dichromate
% Al: _____________
4)
Chloric acid
% O: _____________
% O: _____________
Formula: _______________________
% Cr: _____________
% O: _____________
Formula: _______________________
How much chlorine can be recovered from 175 grams of chloric acid?
Formula and Molecular Weights Practice
Directions: Determine the formula weight of each of the following compounds:
1. N2O5
Name: ________________________________________
2. FeCO3
Name: ________________________________________
3. Ca(C2H3O2)2
Name: ________________________________________
4. (NH4)3PO3
Name: ________________________________________
5. sodium nitrite
Formula: ______________________________________
6. copper (II) thiocyanate
Formula: ______________________________________
7. disilicon hexabromide
Formula: ______________________________________
Percentage Composition Worksheet
Give the % composition of all elements in these compounds. Show ALL work!
1) ammonium sulfite
% N = __________
Formula: ___________________
% H = __________
2) aluminum acetate
% Al = __________
% S = __________
% O = __________
Formula: ____________________
% C = __________
3) beryllium nitride
%H = __________
% O = __________
Formula: ____________________
% Be = __________
4) copper (II) hydroxide
% Cu = __________
% N = __________
Formula: ____________________
% O = __________
% H = __________
Tougher Percent Composition Problems
1) How much iron can be recovered from 25.0 g of iron (III) oxide?
2) How much phosphorus can be produced from 125.0 g of diphosphorus pentoxide?
3) How much oxygen would be found in 48.4 g of chlorous acid?