CBSE Class 10 Chemistry Acids, Bases and Salts MCQs Set 29

Answer: (d) 10

Explanation:
1. A solution that turns red litmus blue is basic.
2. Basic solutions have pH greater than 7, so 10 is the only possible value.

Teacher's Note:
1. Acids have pH below 7, neutral solutions have pH 7 and bases have pH above 7.
2. Only bases turn red litmus blue.

Answer: (a) acidic

Explanation:
1. In water the cation of the weak base reacts with water and releases H⁺ ions.
2. So the solution has pH less than 7 and is acidic.

Teacher's Note:
1. Strong acid + strong base gives a neutral salt.
2. Strong base + weak acid gives a basic salt.

Answer: (b) a weak acid

Explanation:
1. Carbonic acid is only slightly ionised in water.
2. An acid that ionises to a small extent is a weak acid.

Teacher's Note:
1. Strong mineral acids are HCl, H₂SO₄ and HNO₃.
2. Organic acids and carbonic acid are weak.

Answer: (d) NH₄Cl

Explanation:
1. NH₄Cl is the salt of a strong acid (HCl) and a weak base (NH₃).
2. Its NH₄⁺ ion gives H⁺ ions in water, so the solution is acidic.

Teacher's Note:
1. KCN and CH₃COONa are salts of weak acids and strong bases, so they are basic.
2. NaBr is a neutral salt.

Answer: (a) A-R, B-P, C-S, D-Q

Explanation:
1. Blood is slightly basic and tightly controlled at pH 7.36 – 7.42 (R).
2. Soft drinks have a wide acidic range 2.0 – 4.0 (P), vinegar 2.4 – 3.4 (S) and urine 4.8 – 8.4 (Q).
3. So the match is A-R, B-P, C-S, D-Q.

Teacher's Note:
1. Blood has the narrowest pH range of these.
2. Urine shows the widest range, because it varies with diet.

Answer: (a) CO₂ gas

Explanation:
1. On heating, 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂.
2. So carbon dioxide gas is formed.

Teacher's Note:
1. Sodium carbonate itself does not decompose on heating.
2. This reaction is why baking soda makes cakes rise.

Answer: (a) calcium carbonate

Explanation:
1. Marble, limestone and chalk are different forms of calcium carbonate.
2. So the chemical name is calcium carbonate.

Teacher's Note:
1. The formula of calcium carbonate is CaCO₃.
2. It reacts with acids to give CO₂.

Answer: (d) 3/2

Explanation:
1. Gypsum has 2 molecules of water per formula unit and plaster of Paris has ½.
2. The difference is 2 – ½ = 3/2.

Teacher's Note:
1. Gypsum is CaSO₄·2H₂O and plaster of Paris is CaSO₄·½H₂O.
2. On heating, 1½ water molecules per formula unit are lost.

Answer: (d) all four of them

Explanation:
1. All four molecules have a central atom with an incomplete octet.
2. Each can accept a lone pair of electrons, so each is a Lewis acid.

Teacher's Note:
1. Anhydrous halides of B, Al, Be and Fe are typical Lewis acids.
2. A Lewis acid is an electron pair acceptor.

Answer: (b) Na₂CO₃

Explanation:
1. Anhydrous sodium carbonate is called soda ash.
2. Its formula is Na₂CO₃.

Teacher's Note:
1. The decahydrate is washing soda.
2. NaHCO₃ is baking soda.

Answer: (b) Na⁺, CO₃^2– and H₂O

Explanation:
1. Washing soda is sodium carbonate with 10 molecules of water of crystallisation.
2. It therefore contains Na⁺, CO₃^2– and H₂O.

Teacher's Note:
1. The carbonate part comes from carbonic acid.
2. It has no calcium or sulphate in it.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Acetic acid is ionised only to a small extent in water, so it is a weak acid.
2. The degree of dissociation measures this extent, and a small value is exactly why the acid is weak.
3. So both are true and R explains A.

Teacher's Note:
1. Degree of dissociation = ionised molecules ÷ total molecules.
2. For strong acids it is close to 1.

Answer: (c) H₂SO₄

Explanation:
1. A metal oxide reacts with an acid to give a salt and water.
2. CuSO₄ is the sulphate salt, so the acid is sulphuric acid, H₂SO₄.

Teacher's Note:
1. Metal oxide + acid gives salt + water.
2. The anion of the salt tells you the acid used.

Answer: (a) HF

Explanation:
1. Among the hydrogen halides acid strength increases down the group because the H–X bond becomes weaker.
2. HF has the strongest bond, so it ionises least and is the weakest acid.

Teacher's Note:
1. The order of acid strength is HI > HBr > HCl > HF.
2. HF is a weak acid even though fluorine is the most electronegative element.

Answer: (a) boron has an incomplete octet and an empty p orbital

Explanation:
1. In BF₃ boron has only six electrons in its outer shell, so its octet is incomplete.
2. The empty p orbital can accept a lone pair from a Lewis base such as NH₃.

Teacher's Note:
1. BF₃ is trigonal planar and covalent.
2. Boron has no lone pair in this molecule.

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

Explanation:
1. K₂CO₃, Cs₂CO₃ and CH₃COONa are salts of a strong base with a weak acid, so they hydrolyse to give OH^– and a basic solution.
2. NH₄Cl comes from a strong acid and a weak base, so its solution is acidic.

Teacher's Note:
1. Carbonates and acetates of alkali metals are basic salts.
2. Ammonium salts of strong acids are acidic.

Answer: (d) It is a reducing agent

Explanation:
1. Bleaching powder releases chlorine and oxygen, so it is an oxidising agent and bleaches by oxidation.
2. It is used to disinfect water, so (B) and (C) are correct, and (A) is also true.
3. Calling it a reducing agent is wrong.

Teacher's Note:
1. Bleaching action: HOCl gives HCl + [O], and the nascent oxygen destroys colour.
2. It is used to bleach cotton, linen and wood pulp.

Answer: (a) alkaline

Explanation:
1. Sodium carbonate is the salt of a strong base (NaOH) and a weak acid (H₂CO₃).
2. Its carbonate ion takes H⁺ from water and leaves OH^–, so the solution is alkaline.

Teacher's Note:
1. It turns red litmus blue.
2. That is why washing soda helps remove grease.

Answer: (c) dry slaked lime, Ca(OH)₂

Explanation:
1. Chlorine reacts with dry slaked lime to form bleaching powder.
2. Ca(OH)₂ + Cl₂ gives CaOCl₂ + H₂O.

Teacher's Note:
1. Wet lime gives a different product.
2. Chlorine itself is obtained from the electrolysis of brine.

Answer: (c) In water ammonia forms NH₄⁺ and OH^– ions and the OH^– ions turn litmus blue

Explanation:
1. Dry ammonia has no OH^– ions to show basic behaviour.
2. In water, NH₃ + H₂O gives NH₄⁺ + OH^–, and the OH^– ions turn red litmus blue.

Teacher's Note:
1. A base shows its character only through OH^– in solution.
2. Ammonia is a weak base.

Answer: (b) quick lime, CaO

Explanation:
1. Quick lime is basic and absorbs water, and it does not react with ammonia.
2. Concentrated sulphuric acid would react with ammonia since NH₃ is basic; calcium carbonate and slaked lime do not dry gases effectively.

Teacher's Note:
1. A drying agent must not react with the gas to be dried.
2. Ammonia is basic, so an acidic drying agent cannot be used.

Answer: (d) 2

Explanation:
1. pH = –log[H⁺] = –log(10^–2) = 2.

Teacher's Note:
1. pH = –log of the molar [H⁺].
2. 10^–2 M is a fairly strong acid solution.

Answer: (a) The solution turns pink

Explanation:
1. Lime water is a solution of Ca(OH)₂, which is basic.
2. Phenolphthalein turns pink in basic solutions.

Teacher's Note:
1. Phenolphthalein is colourless in acid and pink in base.
2. Methyl orange turns red in acid.

Answer: (a) magnesium + hydrochloric acid gives magnesium chloride + hydrogen

Explanation:
1. An active metal reacts with a dilute acid to give a salt and hydrogen.
2. Mg + 2HCl gives MgCl₂ + H₂.

Teacher's Note:
1. Metal + dilute acid gives salt + hydrogen.
2. Hydrogen is tested by a burning candle, which burns with a pop sound.

Answer: (b) the heat evolved is then absorbed gradually and the container does not crack

Explanation:
1. Dilution of an acid is highly exothermic.
2. Adding acid slowly to water with stirring spreads the heat and avoids splashing, while adding water to acid can generate heat suddenly and crack the glass.

Teacher's Note:
1. Rule: add acid to water, with stirring.
2. Never add water to concentrated acid.

Answer: (d) rain water contains dissolved acids such as carbonic acid which give ions

Explanation:
1. Pure distilled water has almost no ions.
2. Rain water dissolves CO₂ and other gases and forms acids that give H⁺ ions, so it conducts.

Teacher's Note:
1. Conduction needs free ions.
2. Rain water is slightly acidic, with a pH around 5.6.

Answer: (d) it absorbs moisture from the air and sets into hard gypsum

Explanation:
1. Plaster of Paris combines with water to form gypsum.
2. If it is exposed to moist air it slowly sets into a hard mass and becomes useless.

Teacher's Note:
1. Keep it dry until the time of use.
2. Setting is the reaction with water.