More practice questions

4. Q4. Magnesium is 78.99% ^²⁴Mg (23.985 u), 10.00% ^²⁵Mg (24.986 u) and 11.01% ^²⁶Mg (25.983 u). Find its average atomic mass.

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   0.7899 × 23.985 + 0.1000 × 24.986 + 0.1101 × 25.983 = 18.946 + 2.499 + 2.861 ≈ 24.31 u.

5. Q5. Gallium has two isotopes, ^⁶⁹Ga (68.926 u) and ^⁷¹Ga (70.925 u). Its average atomic mass is 69.723 u. Find the percentage of ^⁶⁹Ga.

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   Let the fraction of ^⁶⁹Ga be x: 68.926x + 70.925(1 − x) = 69.723. So 70.925 − 1.999x = 69.723 and x = 1.202/1.999 ≈ 0.601, that is, about 60.1% ^⁶⁹Ga (and 39.9% ^⁷¹Ga).

6. Q6. Find the molecular mass of sulphuric acid, H₂SO₄ (H = 1.008, S = 32.06, O = 16.00 u).

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   2 × 1.008 + 32.06 + 4 × 16.00 = 2.016 + 32.06 + 64.00 = 98.076 u, about 98.08 u.