Quick Notes Percentage Mass

• Percentage by mass refers the percentage of a compound’s mass that is made up of each atom type.
• Empirical formulas can be determined by using the percentage by mass of each atom type within a compound.
• For example, in sodium chloride
• Percentage by mass of sodium (Na) = 39.4% and percentage by mass of chlorine (Cl) = 60.6%
• in 100g, there is 39.4g of Na and 60.6g of Cl
• 39.4g of Na = 1.71 moles (mass/Ar = 39.4 / 23)
• 60.6g of Cl = 1.71 moles (mass/Ar = 60.6 / 35.5)
• Molar ratio = 1 : 1
• Empirical Formula = NaCl

Full Notes Percentage Mass

For how to use Percentage by mass data to find a substances empirical formula, see Empirical Formula.

Composition by mass refers to the amount of a compound’s mass that is made up of each atom type, this is usually expressed as a percentage and is called percentage by mass.

For example, if you take methane (CH4).

In one molecule of methane, there is 1 carbon atom and 4 hydrogen atoms. The relative atomic mass of carbon is 12 and hydrogen 1. This means the relative molecular mass of methane will be 16.

1 x C = 12

4 x H = 4

CH4 = 16

We know that the value of relative atomic mass is the same as the molar mass (mass per mole) of each element. .

This means that 1 mole of methane will have a mass of 16g.

Of this 16g, 12g will be carbon (see above) and 4g will be hydrogen.

Expressed as a percentage, the mass of carbon in the methane will be 75%.

12/16 x 100 = 75

The mass of hydrogen in the methane will be 25%.

4/16 x 100 = 25

These percentages will always be the same, regardless of the mass or moles of methane being analysed.

100g of methane will contain 75g of carbon, as 75% of 100 = 75g.

200g of methane will contain 50g of hydrogen, as 25% of 200 = 50g.

## Percentage by Mass of Water in Hydrated Crystals

Percentage by masses can also refer to the mass of a particular substance within a mixture or compound. A common example of this is with hydrated crystals.

Hydrated crystals contain water that is ‘trapped’ inside the solid crystal structure, called ‘water of crystallisation’. The moles of water trapped in 1 mole’s worth of salt are shown using •.

If a sample of hydrated copper sulfate has the formula CuSO4•22O, it means that for every 1 mole of CuSO4 in the solid crystal, there are 2 moles of H2O. To find the percentage by mass of water in the crystal, we need to know the relative mass of the whole hydrated crystal formula and the relative mass of the water within it.

CuSO4•2H2O has a relative mass of 195.5

Ar of Cu = 63.5, S = 32, O = 16 and H = 1

(63.5 + 32 + (16 x 4)) + 2 x ((1 x 2) + 16) = 195.5

The relative mass of water (2H2O) in the compound = 36

Mr of CuSO4 = 63.5 + 64 + 32 = 159.5

Mr of hydrated CuSO4 = 195.5 (see above)

Mr from H2O = 195.5 - 159.5 = 36

Ar of O = 16 and H = 1

36 / (16 + (2 x 1)) = 2

This means that 1 mole of CuSO4•2H2O has a mass of 195.5 g and within this, 36g come from water (2 x H2O).

Percentage by mass = mass of water / mass of compound x 100

= (36 / 195.5) x 100 = 18.4%

This means, for any mass of sample, 18.4% of the sample will be water (H2O).