CBSE Class 10 Chemistry Acids, Bases and Salts MCQs Set 35

Answer: (c) Antacid

Explanation:
1. Indigestion is caused by excess hydrochloric acid in the stomach.
2. An antacid is a mild base such as magnesium hydroxide, which neutralises the excess acid.

Teacher's Note:
1. Antibiotics kill bacteria and analgesics relieve pain.
2. Milk of magnesia is a common antacid.

Answer: (c) 0.0001

Explanation:
1. pH = –log[H⁺] = 4, so [H⁺] = 10^–4 M.
2. HCl is a strong acid and ionises completely, so its molarity is 10^–4 M = 0.0001 M.

Teacher's Note:
1. [H⁺] = 10^–pH.
2. For a strong monobasic acid, molarity equals [H⁺].

Answer: (a) acidic

Explanation:
1. pH = –log(10^–5) = 5, which is less than 7.
2. So the solution is acidic.

Teacher's Note:
1. [H⁺] greater than 10^–7 M means an acidic solution.
2. [H⁺] = 10^–7 M is neutral.

Answer: (c) A is true, but R is false.

Explanation:
1. A Bronsted base accepts a proton using a lone pair, so it can donate an electron pair and is a Lewis base: A is true.
2. A Bronsted acid such as HCl donates a proton; the acid itself is not an electron-pair acceptor. So R is false.

Teacher's Note:
1. A Lewis acid accepts a pair of electrons; a Bronsted acid donates H⁺.
2. Every Bronsted base is a Lewis base, but the reverse is not always true.

Answer: (d) gypsum

Explanation:
1. Gypsum, CaSO₄·2H₂O, is heated at 373 K.
2. It loses water to form calcium sulphate hemihydrate, which is plaster of Paris.

Teacher's Note:
1. Heating must be controlled at 373 K.
2. Overheating gives anhydrous CaSO₄, which does not set.

Answer: (d) plaster of Paris

Explanation:
1. Plaster of Paris sets to a hard solid (gypsum) when mixed with water.
2. It is used to hold fractured bones in position.

Teacher's Note:
1. Washing soda and baking soda do not set into a hard mass.
2. Plaster of Paris is also used for making toys and smooth wall surfaces.

Answer: (d) NaCl, NH₃, CaCO₃

Explanation:
1. The Solvay process uses brine (NaCl solution), ammonia and limestone (CaCO₃, the source of CO₂).
2. The other three options contain substances that are products or by-products, not the starting materials.

Teacher's Note:
1. NH₄Cl is formed and NH₃ is recovered and reused.
2. Limestone gives CO₂ on heating and lime for recovering ammonia.

Answer: (a) (i) and (iii) only

Explanation:
1. Milk of magnesia is Mg(OH)₂ and ammonia solution is NH₄OH, both bases.
2. Vinegar contains acetic acid and lemon juice contains citric acid, so both are acidic.

Teacher's Note:
1. Milk of magnesia is used as an antacid.
2. Household acids: vinegar, lemon juice, tamarind.

Answer: (c) brine

Explanation:
1. A concentrated solution of common salt in water is called brine.
2. It is the raw material for the chlor-alkali process.

Teacher's Note:
1. Brine electrolysis gives NaOH, Cl₂ and H₂.
2. Seawater is a natural source of salt.

Answer: (d) acetic acid

Explanation:
1. HCl, H₂SO₄ and HNO₃ are mineral acids which ionise almost completely in water.
2. Acetic acid is ionised only to a small extent, so it is the weak acid.

Teacher's Note:
1. In general mineral acids are strong and organic acids are weak.
2. A weak acid shows an equilibrium between molecules and ions.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. An acid HA that loses H⁺ leaves the anion A^–, which is its conjugate base, so A is true.
2. An anion does carry a negative charge, so R is true, but it is not the reason why the anion is the conjugate base.

Teacher's Note:
1. The conjugate base is defined by the loss of a proton.
2. The negative charge is only a result of that loss.

Answer: (b) acetic acid

Explanation:
1. Vinegar is a dilute solution of acetic acid in water.
2. So acetic acid is the acid in vinegar.

Teacher's Note:
1. Formic acid is found in ant and nettle stings.
2. Vinegar is acidic, with a pH near 3.

Answer: (a) CO

Explanation:
1. Zn²⁺ is a cation, SO₃ has an electron-poor sulphur and SiCl₄ can expand its octet; all three accept electron pairs.
2. Carbon monoxide has a lone pair on carbon and acts as an electron-pair donor, so it is not a Lewis acid.

Teacher's Note:
1. All cations are Lewis acids.
2. CO is a well-known ligand, which means it donates electrons.

Answer: (a) (i) and (iii) only

Explanation:
1. In (i) NH₃ donates a lone pair to BF₃, forming a dative bond.
2. In (iii) water donates a lone pair to H⁺.
3. Reaction (ii) is a transfer of electrons between atoms, not a sharing of an electron pair, so it is not Lewis acid-base.

Teacher's Note:
1. A Lewis acid-base reaction forms a coordinate (dative) bond.
2. Metal plus non-metal combining to form ions is a redox reaction.

Answer: (a) (i), (ii) and (iv)

Explanation:
1. Glucose and ethanol do not give ions in water, so their solutions do not conduct.
2. Distilled water has almost no ions and is a very poor conductor.
3. Acetic acid ionises partly and conducts weakly.

Teacher's Note:
1. A solution conducts only if it contains free ions.
2. Acids and bases give ions in water, sugar and alcohol do not.

Answer: (a) The relative strength of acids and bases is explained quantitatively by this concept

Explanation:
1. The Lewis concept covers all coordinate-bond forming reactions (B) and treats all cations as acceptors (C), and O^2– has lone pairs to donate (D).
2. It does not give a quantitative scale of acid or base strength, so (A) is the incorrect statement.

Teacher's Note:
1. Strength scales come from Ka and Kb, which belong to the Arrhenius and Bronsted ideas.
2. Lewis theory is the most general of the three.

Answer: (b) Its H⁺ concentration becomes 10 times greater and it becomes more acidic

Explanation:
1. [H⁺] = 10^–pH M, so at pH 3 it is 10^–3 M and at pH 2 it is 10^–2 M.
2. The H⁺ concentration increases 10 times, so the solution becomes more acidic.

Teacher's Note:
1. Lower pH means higher [H⁺] and more acidity.
2. Each pH unit means a factor of 10.

Answer: (c) Dry HCl gas does not release H⁺ ions; the acid property appears only in water

Explanation:
1. Hydrogen ions are produced only when HCl dissolves in water: HCl + H₂O gives H₃O⁺ + Cl^–.
2. Dry HCl gas does not ionise, so it shows no acidic character; the aqueous solution is hydrochloric acid.

Teacher's Note:
1. Water is needed to separate the H⁺ from the acid molecule.
2. Dry HCl gas does not change dry blue litmus paper.

Answer: (b) acids give H⁺ ions in water, while alcohol and glucose do not give H⁺ ions

Explanation:
1. The acidic character comes from H⁺(aq) ions.
2. HCl and H₂SO₄ ionise in water to give H⁺, while alcohol and glucose contain hydrogen but do not release it as H⁺.

Teacher's Note:
1. Having hydrogen in the formula does not make a compound an acid.
2. The hydrogen must be released as H⁺ in water.

Answer: (b) washing soda crystals

Explanation:
1. Washing soda crystals lose water of crystallisation in air and crumble to a powder.
2. Anhydrous calcium chloride and concentrated sulphuric acid gain water, and common salt has no water of crystallisation.

Teacher's Note:
1. Efflorescence: loss of water. Deliquescence: absorbing water and dissolving.
2. Washing soda is a standard example.

Answer: (c) C < E < D < A < B

Explanation:
1. [H⁺] = 10^–pH, so the higher the pH, the lower the [H⁺].
2. Arranging pH in decreasing order 12, 9, 7, 4, 2 gives C, E, D, A, B.

Teacher's Note:
1. Highest pH has the lowest [H⁺].
2. Use the pH scale as an inverse scale.

Answer: (a) It turns red, because FeCl₃ solution is acidic

Explanation:
1. FeCl₃ is the salt of a strong acid (HCl) and a weak base (Fe(OH)₃).
2. Its solution is acidic because of hydrolysis, so blue litmus turns red.

Teacher's Note:
1. Salts of strong acid and weak base give acidic solutions.
2. Sodium sulphate solution, in contrast, does not change litmus.

Answer: (b) it loses its water of crystallisation

Explanation:
1. Blue vitriol is hydrated copper sulphate, CuSO₄·5H₂O.
2. On heating it loses its five molecules of water and turns into white anhydrous CuSO₄; the blue colour returns on adding water.

Teacher's Note:
1. The blue colour belongs to the hydrated crystal.
2. This colour change is used as a test for water.

Answer: (d) The organic acids in them react with the metal and form poisonous compounds

Explanation:
1. Curd and sour foods contain organic acids such as lactic acid.
2. They slowly react with copper and brass to form toxic salts, which spoil the food and are harmful.

Teacher's Note:
1. Keep acidic foods in glass, steel or plastic vessels.
2. Metal + acid gives a salt.

Answer: (b) CO₂ is evolved and sodium carbonate is formed: 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂

Explanation:
1. Sodium hydrogencarbonate decomposes on heating.
2. 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂, so carbon dioxide gas is liberated.

Teacher's Note:
1. The sodium carbonate formed is the first step of the Solvay process.
2. This is also how a cake rises.

Answer: (a) the alkaline milk has to be neutralised by the lactic acid first

Explanation:
1. Baking soda makes the milk slightly alkaline.
2. The acid produced during setting is first used up to neutralise the base, so curd forms slowly; the alkaline pH also delays souring.

Teacher's Note:
1. A slightly alkaline pH keeps the milk fresh for longer.
2. Acid + base: neutralisation.

Answer: (b) acidic

Explanation:
1. These substances are bases and neutralise soil acidity.
2. The treatment brings the soil pH closer to the range plants prefer.

Teacher's Note:
1. Excess acid in soil (acid rain) harms crops.
2. Lime is cheap and basic.