Calculating Percent Composition
by Mass
Learning Goal: to be able to calculate
the percentages of elements in a
pure substance
Why bother?
Mining
Agriculture
Medicine
How does it work?
• Given a chemical formula you can calculate
the percent of each element
• Step 1: Calculate the total mass of each
element in the compound
• Step 2: Calculate the molar mass of
compound
• Step 3: Calculate percentage composition by
mass of compound
Example: CO2
• Step 1: Calculate the total mass of each
element in the compound
• Step 2: Calculate the molar mass of
compound
• Step 3: Calculate percentage composition by
mass of compound
How does it work?
• Given a chemical formula you can calculate the percent
of each element
• E.g. CO2 (use two decimal places)
• Step 1: Calculate the total mass of each element in
the compound
– MC = 12.01 g/mol
– MO = 2(16.00) g/mol = 32.00 g/mol
• Step 2: Calculate the molar mass of compound
– MCO2 = (12.01 + 32.00) g/mol
– MCO2 = 44.01 g/mol
• Step 3: Calculate percentage composition by mass of
compound
Example: K2SO4
Old MacDonald has a problem
• He is currently putting 55 kg of ammonium nitrate fertilizer per acre on his fields
but his friend in the agricultural sciences department at the University of Manitoba
tells him that he needs to put on a specific amount of nitrogen. First he needs to find
out how much nitrogen he has been putting on his fields. Can you help him?
Arnold has a problem!!!
• Lately he hasn’t been able to lift as much weight as previously. After visiting
Dr. Weiderstumpff he is told that he has an iron deficiency and needs to take an iron
supplement (Iron(II)sulfate) but he must not exceed 45 mg per day. How much should
he take?
Work in pairs to solve the problem!
Dr. Penner has a problem!!!
Gatorade saved my life: here’s the story…..
How much sodium chloride
should I put in one container of
homemade Gatorade?
Take note of
These numbers
http://www.nutritionrx.ca/homemade-sports-drink/