Question

How much nitrogen must be added to a 20% nitrogen solution to obtain 100 Kg of 40% nitrogen solution?

(a) 10 Kg

(b) 25 Kg

(c) 50 Kg

(d) 75 Kg

This question was posed to me during an interview.

I'd like to ask this question from Solving Material Balance Problems for Single Units topic in portion A General Strategy for Solving Material Balance Problems and Solving Material Balance Problems for Single Units of Basic Chemical Engineering

Answer 1

The correct choice is (b) 25 Kg

Explanation: Let the amount of nitrogen added be x, and amount of 20% nitrogen solution be P. Nitrogen balance: x + 0.2(P) = 0.4(100), Water balance: 0.8(P) = 0.6(100), => P = 75 Kg, => x = 0.4(100) – 0.2(75), => x = 25 Kg.