Multiple choice

A mixture of $CuSO_4.5H_2O$ and $MgSO_4.7H_2O$ was heated until all the water was driven off. If $5.0$ g of mixture gave $3$ g of anhydrous salt, what was the percentage by mass of $CuSO_4.5H_2O$ in the original mixture?

  1. $74.4$
  2. $70$
  3. $80$
  4. $90$
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A Correct answer
Explanation

Let x be the mass of CuSO4.5H2O and y be the mass of MgSO4.7H2O. The molar masses are approximately 250 g/mol for CuSO4.5H2O (anhydrous 160) and 246 g/mol for MgSO4.7H2O (anhydrous 120). We have x + y = 5 and (160/250)x + (120/246)y = 3. Solving this system yields x approximately 3.72 g, which is 74.4% of 5 g.

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