Lesson: Chapter 7 — Quantification of Elements and Compounds

7.2. Relative molecular mass 9 of 10 Reset answers

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Question 9

What is the formula mass of potassium sulphate (K₂SO₄)? (A_r: O = 16, S = 32, K = 39)

A 135 B 87 C 174 D 109

The right answer is the one marked above.

174. K₂SO₄ = (2 × 39) + 32 + (4 × 16) = 78 + 32 + 64 = 174.

87 comes from taking 39 + 32 + 16 and ignoring both subscripts. The subscript 2 applies to the potassium alone, not to the sulphate.

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