12 Chemistry 2.3 gravimetric analysis CR 07

Gravimetric analysis

- Using changes in mass to analyse composition

- Done by strongly heating the substance and measuring the loss of mass due to water loss

12 Chemistry 2.3 gravimetric analysis CR 07

Percentage Composition

To calculate the percentage composition of a substance you use the formula:%age = mass x/mass total x

100‘x ‘ can be an element, fat,

water, sugar….anything

12 Chemistry 2.3 gravimetric analysis CR 07

Food example

In a 100g pot of yoghurt 45g is water – what is the %age water in yoghurt?

%age water= mass water/mass yoghurt x 100=45/100 x 100 = 45% water

12 Chemistry 2.3 gravimetric analysis CR 07

Chemistry exampleWhat percentage water is there in

CuSO4.5H2O?

%(H2O)

= M(H2O) /M(CuSO4.5H2O)x 100

=90/249.55x100=36%

12 Chemistry 2.3 gravimetric analysis CR 07

The processTo find the % composition of

each element in a compound use the following steps….

1. Use the formula to find the number of moles of each element.

2. Use the molar masses to find the mass of each element

12 Chemistry 2.3 gravimetric analysis CR 07

3. Write the mass of each element as a fraction of the total mass of one mole of the substance, then multiply by 100 to give %age.

4. CHECK (add up %ages – they should add to 100)

12 Chemistry 2.3 gravimetric analysis CR 07

CuSO4.5H2O1. n(Cu) = 1, n(S) = 1, n(O) = 4 +

5 = 9, n(H) = 5 x 2 = 102. From periodic table:

m(Cu) =63.55gm(S) = 32gm(O) = 64g (16 x 4)m(H) = 10g m(O) = 80g (16 x 5)total mass = 249.55g

12 Chemistry 2.3 gravimetric analysis CR 07

3. % Cu = 63.55/249.55 x100 = 25.5%

% S = 32/249.55 x100= 12.8%

% O = 144/249.55 x100= 57.7%

% H = 10/249.55 x100= 4%

12 Chemistry 2.3 gravimetric analysis CR 07

4. Check: 25.5+12.8+57.7+4 = 100

*always check answers