Acids Bases and Salts
NCERT Exemplar Problems
Long Answer Questions
Question 45: A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back.
On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z.
Answer: Calcium carbonate gives carbon dioxide gas when reacts with hydrochloric acid.
CaCO3 + 2HCl ⇨ CaCl2 + CO2 + H2O
Carbon dioxide turns lime water milky when passed through it because of formation of calcium carbonate. When carbon dioxide; so formed; is passed through lime water, lime water turns milky because of the formation of calcium carbonate.
CO2 + Ca(OH)2 ⇨ CaCO3 + H2O
On the electrolysis of brine, chlorine gas is deposited over anode which gives calcium oxychloride (Bleaching powder) on passing over slaked lime. The bleaching powder is used in disinfecting the drinking water.
Ca(OH)2 + Cl2 ⇨ CaOCl2 + H2O
Bleaching powder is used for disinfecting drinking water.
- Metal carbonate ‘X’ is calcium carbonate.
- Solution ‘Y’ is lime water (Calcium hydroxide).
- Gas ‘G’ is chlorine gas.
- Dry ‘Y’ is dry calcium hydroxide (dry slaked lime).
- Compound ‘Z’ is bleaching powder (Calcium oxychloride).
Question 46: A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by–product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution.
Answer: Dry pellets of sodium hydroxide absorb moisture and turn sticky when kept in open which is also a byproduct of Chlor-alkali process.
Non-metallic oxides are acidic in nature, such as carbon dioxide. When carbon dioxide reacts with sodium hydroxide it gives sodium carbonate and water. Since sodium hydroxide is a base and solution of carbon dioxide is an acid, so reaction taking place between them is a neutralization reaction.
2NaOH + CO2 ⇨ Na2CO3 + H2O
Therefore, common base ‘B’ is sodium hydroxide. Reaction between ‘B’ and acidic oxide is neutralization reaction.
Question 47: A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved.
Answer: Calcium belongs to group 2. Calcium sulphate is a white soft substance. It is known as Plaster of Paris, which can be moulded into different shapes by making its dough.
When Plaster of Paris is left for some time in open, it turns into solid mass because of reaction with moisture present in the atmosphere. The solid mass so formed is known as gypsum and cannot be further used for moulding.
CaSO4.0.5H2O + 1.5H2O ⇨ CaSO4.2H2O
The above said group 2 element is calcium and sulpahte of that element is Plaster of Paris. Plaster of Paris shows such behavior because of presence half water molecule as water of crystallization.
Question 48: Identify the compound X on the basis of the reactions given below. Also, write the name and chemical formulae of A, B and C.
(a) X + Zn ⇨ A + H2
Answer: 2NaOH + Zn ⇨ Na2ZnO2 + H2
(b) X + HCl ⇨ B + H2O
Answer: NaOH + HCl ⇨ NaCl + H2O
(c) X + CH3COOH ⇨ C + H2O
Answer: NaOH + CH3COOH ⇨ CH3COONa + H2O
- Compound ‘A’ is sodium zincate (Na2ZnO2)
- Compound ‘B’ is Sodium chloride (NaCl).
- Compound ‘C’ is sodium acetate (CH3COONa)