C5b: Percentage Composition and Empirical Formula

Every compound has a fixed percentage composition by mass and this composition can be used to identify an unknown sample. This item shows how the mole concept and percentage composition can be used to determine the empirical formula of a compound.

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What does an empirical formula give you?
The simplest whole number ratio of each type of atom in a compound.
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What does number of moles =
Mass/Molar Mass
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100 g of calcium carbonate CaCO3 contains 40 g of calcium and 12 g of carbon. What mass of oxygen does it contain?
48g
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What is the empirical formula of ethane? (C2H6)
CH3
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12 g of magnesium reacts with oxygen to produce 20 g of magnesium oxide. What is the percentage by mass of oxygen in magnesium oxide?
40%
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What is the percentage by mass of hydrogen in sodium hydroxide NaOH?
Mr of NaOH = 40, percentage of hydrogen = 1/40 x 100 = 2.5%
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What is the percentage by mass of nitrogen in ammonium nitrate, NH4NO3?
Mr of NH4NO3 = 40, percentage of nitrogen = 28/80 x 100 = 35%
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3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?
S - Ar = 32. Divide mass by Ar = 3.2/32 = 0.1. O - Ar = 16. Divide mass by Ar = 3.2/16 = 0.2. Divide by smallest number = S - 0.1/0.1 = 1, O - 0.2/0.1 = 2 - Empirical formula = SO2
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Card 2

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What does number of moles =

Back

Mass/Molar Mass

Card 3

Front

100 g of calcium carbonate CaCO3 contains 40 g of calcium and 12 g of carbon. What mass of oxygen does it contain?

Back

Preview of the front of card 3

Card 4

Front

What is the empirical formula of ethane? (C2H6)

Back

Preview of the front of card 4

Card 5

Front

12 g of magnesium reacts with oxygen to produce 20 g of magnesium oxide. What is the percentage by mass of oxygen in magnesium oxide?

Back

Preview of the front of card 5
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