Answer: C (57.7%). Using relative atomic masses Cu = 64, C = 12, O = 16, H = 1:
\(M_r(\text{Cu}_2\text{CO}_3(\text{OH})_2) = 2(64) + 12 + 3(16) + 2(16+1) = 128 + 12 + 48 + 34 = 222\).
The mass of copper in one formula unit is \(2 \times 64 = 128\).
\(\%\,\text{Cu} = \dfrac{128}{222} \times 100\% = 57.7\%\).
The other options come from miscounting the oxygen or hydrogen atoms when working out the relative molecular mass (Mᵣ).
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