Which salt is used to prepare Plaster of Paris?

  1. No.20. Which salt is used to prepare Plaster of Paris?

Answer. Calcium sulphate (CaS04-2H2O) is used to prepare plaster of Paris. Formula of plaster of Paris is CaSO4.1/2 H2O

  1. No.1. Define an acid and a base according to Bronsted-Lowry concept and justify with examples that water is an amphoteric compound.

Answer. In 1923, the Danish chemist Bronsted and the English chemist Lowry independently presented their theories of acids and bases on the basis of proton-transfer. According to this concept:

An acid is a substance, molecule or ion that can donate a proton (H+) to another substance.

A base is a substance that can accept a proton (H+) from another substance.

For example, HC1 acts as an acid while NH3 acts as a base:

       HCl(aq) + NH3(aq)  ————— NH4+(aq) + Cl-(aq)

acid     base

Similarly, when HC1 dissolves in water; HCl acts as an acid and H2O as a base.

HCl(aq) + H2O(aq)  H3O(aq) +   Cl-(aq)

Acid     Base Conjugate acid Conjugate base

The substance that can behave as an acid, as well as, a base is called amphoteric compound.

According to Bronsted- Lowry concept, acid and base always work together to transfer a proton. That means a substance can act as an acid (proton donor) only when another substance simultaneously behaves as a base (proton acceptor). Hence, a substance can act as an acid as well as a base, depending upon the nature of the other substance. For example, H2O acts as a base when it reacts with CH3COOH

CH3COOH(aq)+ H2O(aq)  ————>    CH3COO-(aq) +    H3O+(aq)

Acid           Base               Conjugate base Conjugate acid

While water acts as an acid when it reacts with ammonia

H2O(1) + NH3(aq)  ———>  NH4+(aq)+      OH-(aq)

Acid     Base            Conjugate acid   Conjugate base

According to above two examples, water is an amphoteric compound because it acts as an acid as well as a base.

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