What are the approximate percentages by mass of nitrogen and hydrogen respectively in urea?
- A.43% and 5%
- B.47% and 5%
- C.43% and 7%
- D.47% and 7%
Correct answer
Explanation
The correct answer is D, 47% and 7%. Urea, CO(NH₂)₂, has a molar mass of 60 g, of which 28 g is nitrogen and 4 g is hydrogen.
One molecule of urea has one carbon (12), one oxygen (16), two nitrogen atoms (2 × 14 = 28) and four hydrogen atoms (4 × 1 = 4), giving 60. Nitrogen is 28/60, about 46.7%, which rounds to 47%; hydrogen is 4/60, about 6.7%, which rounds to 7%. This high nitrogen share, about 46%, is why urea is the most widely used nitrogen fertiliser in India.
A is wrong because both figures are too low.
B is wrong because the hydrogen figure of 5% is too low.
C is wrong because the nitrogen figure of 43% is too low.
Exam tip: urea CO(NH₂)₂ = molar mass 60, about 46% nitrogen, the richest common solid nitrogen fertiliser.
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