Percent Composition
% Composition from the Formula
• % mass = mass of element in 1 mole of compound
molar mass of compound
• Then multiply by 100 to get %
• Propane (C3H8) is one of the compounds obtained
from petroleum. Calculate the percent composition
of propane.
• Need to find mass of each element in 1 mole of the
compound & mass of the 1 mole of the compound
• Mass of C in one mole of C3H8= 36.033 g
• Mass of H in one mole of C3H8= 8.064 g
• Molar mass of C3H8= 44.097 g
• % of C = mass of C
mass of propane
x 100
(36.033 g/44.097 g) x 100 = 81.713%
• % of H = mass of H
mass of propane
x 100
(8.064 g/44.097 g) x 100 = 18.29 %
• Results should add up to approximately
100%.
• If not, you did something wrong!
Using % composition
• Can determine how much of a substance can be
produced.
• For example, how many grams of hydrogen are
contained in a 100 g sample of propane?
• How many grams of hydrogen are contained in a
39.0 g sample of propane?
• How many grams are carbon?
Examples
• Determine the percent composition of Al(PO4).
Lab Directions
• Complete “Day 1” Instructions
• Safety!
• Place a sheet of paper with your names
underneath your beaker on back lab table