Question 1

Assertion (A): Glucose (C₆H₁₂O₆) and acetic acid (C₂H₄O₂) have the same empirical formula.
Reason (R): The empirical formula gives only the simplest whole-number ratio of atoms, so different molecular formulas can share the same empirical formula.
Choose the correct option.

Accepted Answer

Both A and R are true and R is the correct explanation of A. For glucose the ratio C : H : O = 6 : 12 : 6 = 1 : 2 : 1, and for acetic acid 2 : 4 : 2 = 1 : 2 : 1; both reduce to the empirical formula CH₂O, so A is true. Because the empirical formula keeps only the simplest whole-number ratio (not the actual atom count), several molecules can share it — so R is true and correctly explains A.

Question 2

Which of the following nitrogen compounds has the HIGHEST mass percentage of nitrogen? (N = 14, H = 1, C = 12, O = 16, S = 32, Cl = 35.5)

Accepted Answer

Urea, CO(NH₂)₂. Compute mass % N for each: urea (M = 60) = 28 ÷ 60 = 46.67%; ammonium nitrate (M = 80) = 28 ÷ 80 = 35%; ammonium chloride (M = 53.5) = 14 ÷ 53.5 = 26.17%; ammonium sulphate (M = 132) = 28 ÷ 132 = 21.21%. Urea has the highest nitrogen content, which is why it is a preferred fertiliser.

Question 3

A compound of carbon and hydrogen has the empirical formula CH and a molar mass of 78 g mol⁻¹. Its molecular formula is (C = 12, H = 1):

Accepted Answer

C₆H₆. Empirical formula mass of CH = 12 + 1 = 13. n = molar mass ÷ empirical formula mass = 78 ÷ 13 = 6. Molecular formula = 6 × CH = C₆H₆ (benzene). The value C₂H₂ (acetylene) corresponds to n = 2 and a molar mass of 26, not 78.

Question 4

In which of the following compounds is the mass percentage of oxygen the GREATEST? (H = 1, C = 12, O = 16, S = 32)

Accepted Answer

H₂O. Mass % O: H₂O = 16 ÷ 18 = 88.89%; CO₂ = 32 ÷ 44 = 72.73%; CO = 16 ÷ 28 = 57.14%; SO₂ = 32 ÷ 64 = 50%. Water has the greatest oxygen percentage because its oxygen atom is paired with two very light hydrogen atoms, so oxygen dominates the total mass.

Question 5

An organic compound on analysis gives 54.5% carbon and 9.1% hydrogen, the rest being oxygen. Its empirical formula is (C = 12, H = 1, O = 16):

Accepted Answer

C₂H₄O. Oxygen % = 100 − 54.5 − 9.1 = 36.4%. Moles in 100 g: C = 54.5 ÷ 12 = 4.54, H = 9.1 ÷ 1 = 9.1, O = 36.4 ÷ 16 = 2.28. Divide by smallest (2.28): C = 2, H = 4, O = 1 → empirical formula C₂H₄O. Forgetting to find oxygen by difference is the usual source of error here.

Question 6

Hydrogen peroxide is found to contain 5.9% hydrogen and 94.1% oxygen by mass. Its empirical formula is (H = 1, O = 16):

Accepted Answer

HO. Moles: H = 5.9 ÷ 1 = 5.9, O = 94.1 ÷ 16 = 5.88. The ratio H : O = 1 : 1, so the empirical formula is HO. (The molecular formula is H₂O₂, twice the empirical unit — but the empirical formula asks only for the simplest ratio, which is HO, not H₂O₂.)

Question 7

What mass of oxygen is present in 90 g of water (H₂O)? (H = 1, O = 16)

Accepted Answer

80 g. Molar mass of H₂O = 18, so 90 g of water = 90 ÷ 18 = 5 mol, containing 5 mol of oxygen atoms = 5 × 16 = 80 g. Equivalently, mass of O = 90 × (16 ÷ 18) = 80 g. The value 10 g is the mass of hydrogen (5 × 2), and 53.3 g wrongly uses the oxygen fraction of glucose.

Percentage Composition & Formulae — IISER Chemistry MCQs with Solutions

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