Biomolecules: JEE Main Chemistry Question with Solution
Uracil is a base present in RNA with the following structure. % of N in uracil is
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Hint 1 of 3
What is the correct molecular formula of uracil based on its structure?
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Correct answer
The percentage of nitrogen in uracil is 25% (or 25).
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Solution
StepWorking
01given
Uracil is a pyrimidine base found in RNA. Its chemical structure has a six-membered ring. This ring contains 2 nitrogen atoms, 4 carbon atoms, 2 carbonyl oxygens, and 4 hydrogen atoms.
02find
The percentage by mass of nitrogen (N) in uracil.
03strategise
Determine the molecular formula of uracil as C4H4N2O2. Calculate the molar mass of uracil using standard atomic masses: C = 12, H = 1, N = 14, O = 16 g/mol. Then calculate the mass percentage of N using: % N = (Total mass of N / Molar mass of Uracil) * 100.
04execute
Molar mass of uracil = (4 * 12) + (4 * 1) + (2 * 14) + (2 * 16).
This equals 48 + 4 + 28 + 32 = 112 g/mol.
Mass of nitrogen = 2 * 14 = 28 g/mol.
% N = (28 / 112) * 100 = 25%.
✓verify
28 / 112 = 1/4 = 0.25, which corresponds to exactly 25%.
The molecular formula is C4H4N2O2.
Its degree of unsaturation = 4 + 1 - (4/2) + (2/2) = 4.
This matches one ring and 3 double bonds (two C=O and one C=C).
This confirms the formula is correct.
Hints that build this answer step by step
What is the correct molecular formula of uracil based on its structure?
C4H4N2O2
What is the molar mass of uracil (C4H4N2O2) using standard atomic weights (C=12,H=1,N=14,O=16)?
112 g/mol
What is the mass percentage of nitrogen in uracil?
How do we count the hydrogens on the ring carbons?
Each carbon atom forms 4 covalent bonds. The two carbonyl carbons already have 4 bonds (two to oxygen, two to ring atoms). The other two carbons share a double bond and one single bond, so each has one implicit C-H bond.
