CBSE Class 12 Chemistry sample paper 2025-26: question 29
From the official CBSE sample question paper for 2025-26, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.
Q29 · 4 marks · Case-basedCompetency-basedElectrochemistry
Conductivity of Aqueous Solutions
Electrical conductivity is based on the flow of ions. Slightly ionized substances are weak electrolytes. Weak acids and bases would be categorized as weak electrolytes because they do not completely dissociate in solution.
Highly ionized substances are strong electrolytes. Strong acids and salts are strong electrolytes because they completely ionize in solution. The ions carry the electric charge through the solution thus creating an electric current. The current, if sufficient enough, will light one or both LEDs on a conductivity meter, shown at right.
The meter has a 9V battery, two parallel copper electrodes and two LED's – one green and one red. The conductivity of a solution can be tested by switching the meter on and dipping the copper electrodes in the solution to be tested.
Substances that do not conduct an electric current are called non-electrolytes. Non-electrolytes do not ionize; they do not contain moveable ions. The LEDs of a conductivity meter will not light because there are no ions to carry the electric current.
The table given below is a guide to the possible conductivity measurements
| Scale | Red LED | Green LED | Conductivity |
|---|---|---|---|
| 0 | off | off | low or none |
| 1 | dim | off | low |
| 2 | medium | off | medium |
| 3 | bright | dim | high |
| 4 | very bright | medium | very high |
source: https://chem.libretexts.org
Based on the information provided above, answer the following questions:
- I.1 mark
Is it possible to identify whether the given solution is 1 M NaOH or 1 M HCl using the conductivity meter? Justify your answer.
- II.1 mark
What is the possible pH value of solution if the glow of green LED is medium and the red LED glows very brightly?
(i) 1 (ii) 13 (iii) 5 (iv) 8
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv) OR
II.1 markWrite down the observations if the conductivity meter is dipped in distilled water.
- III.2 marks
Refer to the table given below and draw the molar conductivity vs. concentration curve for solution A and B.
Solution Red LED Green LED A bright dim B dim off
Figure in the paper: Photograph of a hand-held conductivity meter (circuit board with a red LED and a green LED labelled) with two copper electrodes dipped in a beaker of solution. See it in the official paper.
Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.
Hints
Open one hint at a time and try again before the next.
Hint 1
The meter responds only to how many ions are present, not to whether the solution is acidic or basic.
Hint 2
Very high conductivity needs a strong acid or strong base, so which pH values fit?
Hint 3
Recall the NCERT graph of Λm against √c for KCl and for CH₃COOH.
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.
- 1 mark
I. No. 1 M NaOH (strong base) and 1 M HCl (strong acid) are both strong electrolytes that ionise completely. Both give very high conductivity (red LED very bright, green LED medium, scale 4). The meter only shows the amount of conductivity, so it cannot tell the two apart.
- 1 mark
II. Red very bright and green medium means scale 4, very high conductivity, so the solution is a strong acid or strong base. Possible pH values are 1 and 13, i.e. option (a) (i) and (ii).
OR (distilled water): both LEDs stay off (scale 0). Distilled water has very few ions, so its conductivity is negligible.
- 1 mark
III. Solution A (red bright, green dim, high conductivity) is a strong electrolyte; B (red dim, green off, low conductivity) is a weak electrolyte.
Curve for A: is high and decreases only slightly and almost linearly as concentration () increases (), so the line can be extrapolated to .
- 1 mark
Curve for B: is low at higher concentration but rises very steeply as concentration approaches zero, because the degree of dissociation increases on dilution. The curve is not linear and cannot be extrapolated. (Axes: on y-axis, on x-axis.)
Answer: I. No, both are strong electrolytes giving the same high reading. II. (a) (i) and (ii), pH 1 and 13 [OR distilled water: both LEDs off, scale 0]. III. A is a strong electrolyte (Λm falls slightly, linearly with √c); B is a weak electrolyte (Λm rises steeply on dilution).
Where the marks are
- No, with reason that both are strong electrolytes giving the same high conductivity (1 mark)
- Option (a) pH 1 and 13, or both LEDs off for distilled water (1 mark)
- Identifies A as strong electrolyte with correct near-linear curve (1 mark)
- Identifies B as weak electrolyte with correct steep curve (1 mark)
Revise the chapter
- Chapter: Electrochemistry (notes, formulas, where marks are lost and every past question)
More questions from Electrochemistry
- Sample paper 2023-24, Q1 · 1 markWhich of the following solutions will have the highest conductivity at 298 K?
- Sample paper 2026-27, Q12 · 1 markWhich solution has the highest conductivity?
- 2025 board paper, set 56/1/1, Q2 · 1 markStandard electrode potential for Sn^4+/Sn²+ couple is +0.15 V and that for the Cr³+/Cr couple is –0.74 V. The …
The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.