Explain how the percentage of sulphur in a coal sample is determined, and write its significance. In such a determination, 1.5 g of coal burnt in a bomb calorimeter gave 0.12 g of BaSO₄. Calculate the percentage of sulphur in the coal.

Answer: Sulphur is converted to sulphate and weighed as BaSO₄; \(\%S=\dfrac{32\times\text{wt. of BaSO}_4\times100}{233\times\text{wt. of coal}}\). Here \(\%S=1.10\%\).

Procedure (ultimate analysis):

1. A known weight of coal is burnt completely in a bomb calorimeter in excess oxygen (this can be the same run used to find the calorific value).
2. Sulphur in the coal is oxidised to SO₂/SO₃, which dissolves in the water present to form sulphuric acid (sulphate ions).
3. The inside of the bomb is washed with distilled water and the washings are collected.
4. The washings are acidified with dil. HCl and an excess of BaCl₂ solution is added; sulphate is precipitated as BaSO₄: \(\text{SO}_4^{2-}+\text{Ba}^{2+}\to\text{BaSO}_4\downarrow\).
5. The precipitate is filtered, washed, dried, ignited and weighed.

Calculation basis: 233 g of BaSO₄ contains 32 g of S.

\[\%S=\frac{32}{233}\times\frac{\text{wt. of BaSO}_4}{\text{wt. of coal}}\times100=\frac{32\times0.12\times100}{233\times1.5}=\frac{384}{349.5}=1.10\%\]

Significance:

• Sulphur burns to SO₂ and SO₃, which pollute air and cause acid rain.
• These gases with moisture form acids that corrode boilers, chimneys and other equipment.
• Sulphur in coke used for steel-making passes into the metal and makes it brittle, so metallurgical coal must be low in sulphur.
• Although sulphur adds a little heat on burning, a good coal should have as little sulphur as possible.