JAMB 2013 Physics · Question 29

If a current of 2.5A flows through an electrolyte for 3 hours and 1.8g of a substance is deposited, what is…

Question

If a current of 2.5A flows through an electrolyte for 3 hours and 1.8g of a substance is deposited, what is the mass of the substance that will be deposited if a current of 4A flows through it for 4.8 hours?

A.2.4g
B.3.2g
C.4.6g
D.4.8g

Answer: C. 4.6g

Explanation written with AI and checked against the answer key

Analyze the question This is a Faraday's law of electrolysis problem. The mass deposited depends on the quantity of charge passed (Q = It), and the same substance is used in both cases. Concept Faraday's first law: m = zIt, so m ∝ It (z is constant for the same substance). Therefore: m₁/(I₁t₁) = m₂/(I₂t₂) Solution Case 1: I₁t₁ = 2.5 × 3 = 7.5 A·h, with m₁ = 1.8 g Case 2: I₂t₂ = 4 × 4.8 = 19.2 A·h, with m₂ = ? m₂ = m₁ × (I₂t₂)/(I₁t₁) m₂ = 1.8 × (19.2 / 7.5) m₂ = 1.8 × 2.56 m₂ = 4.608 g ≈ 4.6 g (The hours cancel, so there is no need to convert to seconds.) Answer: C (4.6 g) Tip: When the same substance is deposited, just compare the products I × t as a ratio. Skip finding z or the charge in coulombs, since the units cancel and you save time in the exam. Here, 19.2/7.5 = 2.56, and 1.8 × 2.56 is about 4.6.

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