Question

1) why is plaster of paris is written as CaSO4.1/2 H2O? HOW IS IT POSIBBLE TO HAVE HALF A WATER MOLECULE ATACHED TO CaSO4?

2) WHEN ELECTRICITY IS PASSED THROUGH AN AQUOUS SOLUTION OF SODIUM CHLORIDE WHAT ARE THE THREE PRODUCTS OBTAINED? WHY IS THE PROCESS CALLED CHLOR ALKALI PROCESS?

3) EXPLAIN USES OF WASHING SODA FOR SOFTENING OF HARD WATER?

Answer 1

1. In plaster of paris, two molecules of calcium sulphate share one molecule of water of hydration. Therefore, to write the formula, half a molecule of water is written.

2. The products formed when electricity is passed through an aqueous solution of sodium chloride, the products formed are chlorine gas, hydrogen gas and sodium hydroxide.
The name CHLOR ALKALI comes from the products, chlorine and sodium hydroxide, which is an alkali.

3. Hard water is due to the presence of calcium and magnesium salts like calcium and magnesium sulphates, bicarbonates in water. Washing soda or Na₂CO₃ can precipitate these ions as their insoluble salts, removing them from the water and making the water soft.
Na₂CO₃ + CaSO₄ $\to$ Na₂SO₄ + CaCO₃
Na₂CO₃ + Ca(HCO₃)₂ $\to$ 2NaHCO₃ + CaCO₃