Uracil is a base present in RNA with the following structure. % of N in uracil is ___________
Given:
Molar mass N = 14 g mol⁻¹
O = 16 g mol⁻¹
C = 12 g mol⁻¹
H = 1 g mol⁻¹

Solution

Chemistry ma’am · 3 steps

Read the molecular formula off the structure atom by atom, then take nitrogen's share of the molar mass.

1. Count the atoms. The six-membered ring has 4 carbons and 2 nitrogens. Two carbons carry a C=O, so there are 2 oxygens. The CH=CH gives 2 H and the two N-H groups give 2 more, so the formula is C₄H₄N₂O₂.
2. Molar mass. 4 × 12 + 4 × 1 + 2 × 14 + 2 × 16 = 48 + 4 + 28 + 32 = 112 g mol⁻¹.

   % N = (2 × 14)/112 × 100 = 28/112 × 100 = 25

3. So. Nitrogen makes up 25 % of uracil by mass.

The common slip: 12.5. Counting nitrogen's mass only once (14 instead of 2 × 14) gives 12.5. There are two N atoms in the ring.

The idea to keep. For mass percent, first fix the full molecular formula, including every hidden H on carbon and nitrogen. Then multiply each atom's mass by how many times it appears before dividing.

Answer from the NTA final answer key. Solution by PrepWiser.