Question

If a urea (NH2CONH2) contains 45% (by mass) N2, what is the actual urea content in the sample?

(a) 103.7 kg

(b) 96.4 kg

(c) 9.65 kg

(d) 10.4 kg

The question was posed to me during an online interview.

This intriguing question originated from Expressing Concentration of Solutions in division Solutions of Chemistry – Class 12

Answer 1

The correct answer is (b) 96.4 kg

Explanation: Let us consider 100 kg sample or urea. This is said to contain 45kg N2.

Now, 60kg urea contains 28 kg N2. (Since, Mw of urea = 14 + 2 + 12 + 16 + 14 + 2 = 60kg/kmole)

Then, 1kg urea contains \(\frac{28}{60}\)kg N2.

Using unitary method, x kg urea will contain 45 kg N2.

Solving for x –

x= 45 x \(\frac{60}{28}\) = 96.4 kg.