Atomic & Molecular Masses — IISER Chemistry MCQs with Solutions
Free IISER Chemistry Atomic & Molecular Masses MCQs with step-by-step solutions (7 questions). Part of Some Basic Concepts of Chemistry. Practise online on Prepizo — no login needed.
▶ Practise Atomic & Molecular Masses online (free)
Questions with solutions
Q1 — Atomic & Molecular Masses · medium · theory
Assertion (A): The average atomic mass of naturally occurring chlorine, 35.5 u, is not a whole number.
Reason (R): Natural chlorine is a mixture of the isotopes ³⁵Cl and ³⁷Cl, and its average atomic mass is the weighted mean of the isotopic masses taken in proportion to their relative abundances.
Choose the correct option.
A. A is true but R is false
B. Both A and R are true but R is NOT the correct explanation of A
C. Both A and R are true and R is the correct explanation of A ✓ Correct
D. A is false but R is true
Solution: For chlorine, ³⁵Cl (about 75%) and ³⁷Cl (about 25%) give an average = (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5 u. Because this is a weighted mean of two whole-number isotopic masses, the result is fractional. R therefore correctly explains why A is true.
Q2 — Atomic & Molecular Masses · hard · numerical
Lithium exists as two isotopes, ⁶Li and ⁷Li, and its average atomic mass is 6.94 u. The percentage abundance of the ⁷Li isotope is:
A. 6%
B. 90%
C. 7%
D. 94% ✓ Correct
Solution: Let the fraction of ⁶Li be x. Then 6x + 7(1 − x) = 6.94 ⇒ 7 − x = 6.94 ⇒ x = 0.06. So ⁶Li is 6% and ⁷Li is 1 − 0.06 = 0.94, i.e. 94%. The 6% option is the abundance of the lighter isotope, not the ⁷Li asked for.
Q3 — Atomic & Molecular Masses · hard · numerical
Magnesium occurs as three isotopes: ²⁴Mg (79%), ²⁵Mg (10%) and ²⁶Mg (11%), with isotopic masses 24, 25 and 26 u. Its average atomic mass is:
A. 24.32 u ✓ Correct
B. 24.50 u
C. 24.00 u
D. 25.00 u
Solution: Average atomic mass = (0.79 × 24) + (0.10 × 25) + (0.11 × 26) = 18.96 + 2.50 + 2.86 = 24.32 u. A plain average of 24, 25 and 26 gives 25.00 u, and using only the most abundant isotope gives 24.00 u; both ignore the actual abundances.
Q4 — Atomic & Molecular Masses · medium · numerical
Taking the molecular mass of water as 18 u and 1 u = 1.66 × 10⁻²⁴ g, the mass of a single water (H₂O) molecule is:
A. 2.99 × 10⁻²² g
B. 1.66 × 10⁻²⁴ g
C. 2.99 × 10⁻²³ g ✓ Correct
D. 1.80 × 10⁻²³ g
Solution: Mass of one molecule = (molecular mass in u) × (mass of 1 u) = 18 × (1.66 × 10⁻²⁴ g) = 29.88 × 10⁻²⁴ g = 2.99 × 10⁻²³ g. Forgetting to multiply by 1.66 (using 18 × 10⁻²⁴) gives 1.80 × 10⁻²³ g; a power-of-ten slip gives 2.99 × 10⁻²² g; 1.66 × 10⁻²⁴ g is the mass of just 1 u.
Q5 — Atomic & Molecular Masses · medium · numerical
The formula mass of the sulphate ion, SO₄²⁻ (neglecting the negligible mass of the two extra electrons), using S = 32 u and O = 16 u, is:
A. 64 u
B. 48 u
C. 96 u ✓ Correct
D. 98 u
Solution: SO₄²⁻ has one sulphur and four oxygen atoms: 32 + (4 × 16) = 32 + 64 = 96 u. The mass of the two extra electrons is negligible and is ignored. Using only one oxygen (32 + 16) gives 48 u; counting only the four oxygens gives 64 u; adding two hydrogens as in H₂SO₄ gives 98 u.
Q6 — Atomic & Molecular Masses · medium · numerical
Using C = 12, N = 14, O = 16 and S = 32 (all in u), which of the following pairs of species have the SAME molecular mass?
A. NO and O₂
B. CO₂ and SO₂
C. N₂ and CO ✓ Correct
D. O₂ and N₂
Solution: N₂ = 2 × 14 = 28 u and CO = 12 + 16 = 28 u, so this pair is isobaric (equal mass). The others differ: CO₂ = 44 u but SO₂ = 32 + 32 = 64 u; O₂ = 32 u but N₂ = 28 u; NO = 14 + 16 = 30 u but O₂ = 32 u.
Q7 — Atomic & Molecular Masses · hard · numerical
A metal hydroxide X(OH)₂ has a formula mass of 74 u (O = 16 u, H = 1 u). The atomic mass of the metal X is:
A. 57 u
B. 42 u
C. 40 u ✓ Correct
D. 34 u
Solution: X(OH)₂ contains one X, two O and two H atoms: mass = X + 2(16 + 1) = X + 34. So X = 74 − 34 = 40 u (the metal is calcium, giving Ca(OH)₂). Forgetting the two hydrogens (X + 32 = 74) gives 42 u; using only one hydroxide group (X + 17 = 74) gives 57 u; 34 u is just the mass of the two OH groups.