C2 - Percentage Mass

Calculating percentage mass, Empirical formula of a compound

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Calculating Percentage Mass

The mass of the compound is its relative formula mass (top number in symbol).

What do you need to know to calculate the percentage mass of an element in a compound?

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Calculating Percentage Mass

To calculate the percentage mass of an element in a compound, you need to know:

  • the formula of a compound
  • the relative atomic mass of all the atoms
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Calculating Percentage Mass

You can calculate the percentage mass by using what formula?

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Calculating Percentage Mass

You can calculate the percentage mass by using this formula:

(Relative mass of element in the compound / Relative formula mass of compound)x100

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Calculating Percentage Mass

Example 1

Calculate the percentage mass of magnesium in magnesium oxide, MgO.

Relative mass of magnesium = 24

Relative mass of oxygen = 16

Relative formula mass of MgO =  24+16=40

(Relative mass of element in the compound / Relative formula mass of compound)x100

=24/40 x100 =60%

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Calculating Percentage Mass

Example 2

Calculate the percentage mass of potassium in potassium carbonate, k2CO3

Relative mass of potassium = 39 x 2

Relative mass of carbon = 12

Relative mass of oxygen = 16 x 3

Relative formula mass of K2CO3 = 78 + 12 + 48 = 138

(Relative mass of element in the compound / Relative formula mass of compound)x100

78/138 x100 = 56.5%

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Empirical Formula of a Compound

The empirical formula of a compound is the simplest formula that represents the composition of the compound by mass.

Example

Find the simplest formuls of an oxide of iron produced by reacting 1.12g of iron with 0.48g of oxygen.

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Empirical Formula of a Compound

Identify the mass of the elements in the compound

Masses: Fe  = 1.12,    O = 0.48

Divide these masses by their relative atomic masses

Fe = 1.12/56 = 0.02    O = 0.48/16 = 0.03

Identify the ratio of atoms in the compound

Ratio = 0.02 : 0.03

            x100

            2 : 3

Empirical Formula = Fe2O3

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Comments

Krissie

the top number in the symbol is the relative atomic mass. The relative formula mass is when the atomic masses of each element in a compound are added together

E.g. The relative atomic mass of oxygen is 16, but

The relative formula mass of water =

H20 =

2 x H , 1 x O =

2 x 1 + 1 x 16 = 18.

The relative formula mass of water is 18. This would not be shown on the emprical formula of the compound.

Sorry, I hope that helped a bit :P :)

raeesa

these are really well done :D helpd me a lot thnkyuuh :D

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