Answer the following. (i) Five solutions A, B, C, D and E when tested with pH paper showed pH as 4, 1, 13, 7 and 10 respectively. Which solution is : (1) Strongly acidic (2) Strongly alkaline (3) Weakly acidic (4) Neutral and (5) Weakly alkaline ? Arrange the solutions in increasing order of H^+ ion concentration. (ii) Write the name and formula of (1) an acidic salt and (2) a basic salt giving the name of the parent acid and parent base used to form the salt in each case.

ScienceCBSE Class 10 · Board exam 2024, set 31/3/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 34(a)

Set in the CBSE Class 10 board exam of 2024, paper 31/3/1, section D, question 34(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(a) · 5 marks · Long answerAcids, Bases and Salts

Answer the following.

(i)
Five solutions A, B, C, D and E when tested with pH paper showed pH as 4, 1, 13, 7 and 10 respectively. Which solution is : (1) Strongly acidic (2) Strongly alkaline (3) Weakly acidic (4) Neutral and (5) Weakly alkaline ? Arrange the solutions in increasing order of $H^{+}$ ion concentration.
0 marks
(ii)
Write the name and formula of (1) an acidic salt and (2) a basic salt giving the name of the parent acid and parent base used to form the salt in each case.
0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

pH 7 is neutral; the further below 7, the more acidic; the further above 7, the more alkaline.

Hint 2

A higher H⁺ concentration means a lower pH.

Hint 3

An acidic salt comes from a strong acid with a weak base; a basic salt from a weak acid with a strong base.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(i)(1) Strongly acidic: Solution B (pH 1). pH far below 7 means strongly acidic.
½ mark
(i)(2) Strongly alkaline: Solution C (pH 13).
½ mark
(i)(3) Weakly acidic: Solution A (pH 4), which is below 7 but close to it.
½ mark
(i)(4) Neutral: Solution D (pH 7).
½ mark
(i)(5) Weakly alkaline: Solution E (pH 10).
½ mark
**Increasing order of $H^{+}$ concentration:** The lower the pH, the higher the $[H^{+}]$ . So the order of decreasing pH gives $C (13) < E (10) < D (7) < A (4) < B (1)$ .
½ mark
(ii)(1) Acidic salt: Ammonium chloride, $N H_{4} Cl$ . It is formed from a strong acid and a weak base, so its solution has pH < 7.
½ mark
Parent acid: hydrochloric acid, $HCl$ . Parent base: ammonium hydroxide, $N H_{4} OH$ .
½ mark
(ii)(2) Basic salt: Sodium carbonate, $N a_{2} C O_{3}$ . It is formed from a weak acid and a strong base, so its solution has pH > 7.
½ mark
Parent acid: carbonic acid, $H_{2} C O_{3}$ . Parent base: sodium hydroxide, $NaOH$ .
½ mark

Answer: (i) Strongly acidic B; strongly alkaline C; weakly acidic A; neutral D; weakly alkaline E. Order of increasing [H⁺]: C < E < D < A < B. (ii) Acidic salt NH₄Cl (HCl + NH₄OH); basic salt Na₂CO₃ (H₂CO₃ + NaOH).

Where the marks are

Each correct identification of B, C, A, D, E (½ × 5 = 2½ marks)
Correct order C < E < D < A < B (½ mark)
Acidic salt name and formula (½ mark); its parent acid and base (½ mark)
Basic salt name and formula (½ mark); its parent acid and base (½ mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2024: question 34(a)

Hints

Step-by-step solution

Revise the chapter

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