CBSE Class 10 Chemistry Acids, Bases and Salts MCQs Set 31

Answer: (b) HCl

Explanation:
1. Egg-shells are mainly calcium carbonate.
2. A carbonate reacts with an acid to give CO₂, which turns lime water milky.
3. Only HCl is an acid, so the solution contains HCl.

Teacher's Note:
1. Metal carbonate + acid gives salt + water + carbon dioxide.
2. NaCl, LiCl and KCl are neutral salts and do not react with carbonates.

Answer: (a) acidic

Explanation:
1. pH below 7 means an acidic solution.
2. pH = 0 corresponds to [H⁺] = 1 M, a strongly acidic solution.

Teacher's Note:
1. The usual pH scale runs from 0 to 14.
2. pH 0 is the most acidic end.

Answer: (d) strongly basic

Explanation:
1. pH 13 is close to the top of the 0 to 14 scale.
2. A solution with a high pH has a high [OH^–], so it is strongly basic.

Teacher's Note:
1. Values near 1 are strongly acidic and near 13 strongly basic.
2. Values near 6 or 8 indicate weak acidity or basicity.

Answer: (b) Lime stone

Explanation:
1. Limestone is CaCO₃, which decomposes on heating to CaO and CO₂.
2. Slaked lime, quick lime and soda ash do not give CO₂ on heating.

Teacher's Note:
1. Heating limestone is the industrial method of making quick lime.
2. Soda ash (anhydrous Na₂CO₃) is stable to heat.

Answer: (b) gives chlorine on exposure to the atmosphere

Explanation:
1. Bleaching powder reacts with CO₂ and moisture of air and slowly releases chlorine.
2. This is why it smells of chlorine.

Teacher's Note:
1. It is not a simple mixture of chlorine and slaked lime.
2. It is made by passing chlorine over dry slaked lime.

Answer: (c) NaHCO₃

Explanation:
1. Baking soda is sodium hydrogencarbonate.
2. Its formula is NaHCO₃.

Teacher's Note:
1. Na₂CO₃ is soda ash and Na₂CO₃·10H₂O is washing soda.
2. Baking soda is a mild non-corrosive base.

Answer: (b) Na₂CO₃·10H₂O

Explanation:
1. Washing soda is hydrated sodium carbonate with 10 molecules of water of crystallisation.
2. Its formula is Na₂CO₃·10H₂O.

Teacher's Note:
1. Anhydrous Na₂CO₃ is called soda ash.
2. Washing soda is used to soften hard water.

Answer: (b) HOCl, HClO₃ and H₃O⁺

Explanation:
1. The conjugate base is formed when an acid loses H⁺: HCl gives Cl^–, HOCl gives OCl^–, HClO₃ gives ClO₃^– and H₃O⁺ gives H₂O.
2. Cl^– comes only from HCl, so it is not the conjugate base of HOCl, HClO₃ or H₃O⁺.

Teacher's Note:
1. Conjugate base = acid minus one proton.
2. Check each acid by removing one H⁺ and see what remains.

Answer: (c) a normal salt, with no replaceable H⁺ or OH^–

Explanation:
1. ZnSO₄ is formed by complete neutralisation of H₂SO₄ by zinc hydroxide or oxide.
2. It contains neither a replaceable hydrogen (acidic salt) nor an OH^– (basic salt), and it has only one cation, so it is a normal salt.

Teacher's Note:
1. Acidic salts such as NaHSO₄ still contain replaceable hydrogen.
2. A double salt has two different cations, as in alum.

Answer: (a) HCl, HNO₃ and CH₃COOH

Explanation:
1. A monobasic acid has one replaceable hydrogen per molecule.
2. HCl, HNO₃ and CH₃COOH each give one H⁺ (in CH₃COOH only the –COOH hydrogen is replaceable).
3. H₂SO₄, H₃PO₄ and H₂CO₃ are dibasic or tribasic.

Teacher's Note:
1. Count only the hydrogen that is released as H⁺ in water.
2. Acetic acid has four hydrogens but is still monobasic.

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

Explanation:
1. SiF₄ is a Lewis acid because the silicon atom has vacant d orbitals and can accept electron pairs, so A is true.
2. Silicon already has a complete octet in SiF₄, so it is not electron deficient and R is false.

Teacher's Note:
1. Electron deficient species have an incomplete octet, like BF₃.
2. Silicon halides accept electrons by expanding the octet.

Answer: (a) It increases by 4

Explanation:
1. [OH^–] = 0.01 ÷ 10 = 10^–3 M, so pOH = 3 and pH = 11.
2. Pure water has pH 7, so the pH rises by 11 – 7 = 4.

Teacher's Note:
1. Use pH = 14 – pOH for bases.
2. The change is 4 units, not 12 which is the final pH only if [OH^–] were 10^–2 M.

Answer: (a) Apple juice

Explanation:
1. Apple juice contains malic acid, so it is acidic.
2. Soap solution and slaked lime are basic, and lime (CaO) is a basic oxide.

Teacher's Note:
1. Soaps are salts of fatty acids with a strong base, so they are basic.
2. Fruit juices are generally acidic.

Answer: (c) LiOH

Explanation:
1. Basic strength of the alkali metal hydroxides increases down the group as the metal-oxygen bond weakens.
2. LiOH is the weakest and CsOH the strongest.

Teacher's Note:
1. All of these are strong bases but they differ in extent.
2. Li is the smallest ion, so it holds OH^– most strongly.

Answer: (b) [H⁺] decreases and [OH^–] increases

Explanation:
1. Dilution lowers the acid concentration, so [H⁺] decreases.
2. Because [H⁺][OH^–] = 10^–14 stays constant, [OH^–] must rise.

Teacher's Note:
1. The solution becomes less acidic and moves towards pH 7.
2. Dilution never makes an acid basic.

Answer: (a) BF₃ is a Lewis acid with an incomplete octet and accepts the lone pair of NH₃

Explanation:
1. Nitrogen in NH₃ has a lone pair, so it is a Lewis base.
2. Boron in BF₃ has an incomplete octet and accepts the pair, forming the coordinate bond H₃N→BF₃.

Teacher's Note:
1. Donor: ammonia. Acceptor: boron trifluoride.
2. The arrow points from donor to acceptor.

Answer: (a) Na⁺ ions

Explanation:
1. Both compounds are sodium salts, so both contain Na⁺.
2. Neither contains calcium, sulphate or chloride.

Teacher's Note:
1. Both are made from sodium chloride (common salt).
2. Only washing soda has water of crystallisation.

Answer: (b) No, because strength depends on the extent of ionisation, not on concentration

Explanation:
1. The strength of an acid depends on how fully it ionises in water.
2. Acetic acid ionises only slightly, so even a concentrated solution is a weak acid.

Teacher's Note:
1. Concentrated and dilute describe the amount of acid per volume.
2. Strong and weak describe the degree of ionisation.

Answer: (d) CaCO₃

Explanation:
1. A gas that extinguishes a candle and gives effervescence with acid is CO₂, so A is a carbonate.
2. Because calcium chloride is formed, A is CaCO₃: CaCO₃ + 2HCl gives CaCl₂ + H₂O + CO₂.

Teacher's Note:
1. Calcium metal would give hydrogen, which burns with a pop.
2. Calcium hydroxide would give no gas.

Answer: (a) It is used as an antacid because it is a mild base which neutralises excess acid

Explanation:
1. Baking soda, NaHCO₃, is a mild non-corrosive base.
2. It neutralises excess HCl in the stomach, so it is used in antacids; it is also a constituent of baking powder.

Teacher's Note:
1. The base property is used in antacids.
2. The reaction with acids that releases CO₂ is used in baking powder.

Answer: (a) Yes, but [H⁺] is much smaller than [OH^–]

Explanation:
1. In every aqueous solution water ionises slightly, so both H⁺ and OH^– are present, with Kw = [H⁺][OH^–] = 10^–14 at 298 K.
2. A solution is basic because [OH^–] is greater than [H⁺].

Teacher's Note:
1. The two concentrations are equal only in a neutral solution.
2. If one rises, the other falls.

Answer: (b) 1 M NaOH, because it is a strong base

Explanation:
1. 1 M HCl has [H⁺] = 1 M, so its pH is 0.
2. 1 M NaOH has [OH^–] = 1 M, so pOH = 0 and pH = 14, which is higher.

Teacher's Note:
1. Bases have pH above 7 and acids below 7.
2. These two solutions are at the opposite ends of the scale.

Answer: (b) it contains impurities such as MgCl₂ which absorb moisture from the air

Explanation:
1. Common salt contains small amounts of magnesium chloride and other impurities which are deliquescent.
2. They absorb moisture from the humid air, making the salt sticky.

Teacher's Note:
1. Pure sodium chloride does not absorb much moisture.
2. Deliquescent substances absorb water and dissolve in it.

Answer: (b) Zn + H₂SO₄ gives ZnSO₄ + H₂

Explanation:
1. Zinc is above hydrogen in the activity series and displaces it from the dilute acid.
2. The products are zinc sulphate and hydrogen gas, and the equation is balanced as written.

Teacher's Note:
1. Dilute acid gives hydrogen; concentrated acid can give SO₂ instead.
2. Check that atoms balance on both sides.

Answer: (a) A has more H⁺ ions and is acidic; B is basic

Explanation:
1. [H⁺] = 10^–6 M in A and 10^–8 M in B, so A has more H⁺ ions.
2. pH 6 is below 7 (acidic) and pH 8 is above 7 (basic).

Teacher's Note:
1. The solution with the lower pH has more H⁺.
2. pH 7 is neutral at 298 K.

Answer: (a) decrease, because curd contains lactic acid

Explanation:
1. Curd contains lactic acid, so it has more H⁺ ions than fresh milk.
2. More H⁺ means a lower pH, so the pH decreases.

Teacher's Note:
1. Souring of milk makes it more acidic.
2. Acid production lowers the pH.

Answer: (a) A, because HCl is a strong acid and gives more H⁺ ions

Explanation:
1. HCl is almost completely ionised and gives a much higher [H⁺] than acetic acid of the same concentration.
2. The reaction with Mg is faster, so the fizzing of hydrogen is more vigorous in A.

Teacher's Note:
1. Reaction rate with metals depends on [H⁺].
2. Strong acids react faster than weak acids.