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Mass of CuSO₄ you need to prepare a 80% (w/w) solution in 25mL of water is 100g

Explanation:

A solution of 80% (w/w) means you have 80g of solute (CuSO₄, in this case) per 100g of solution (CuSO₄ + water)

Density of water is 1.00g/mL. That means you have 25g of water, thus, to prepare a solution of 80% in mass you need:

80% = Mass CuSO₄ / (Mass CuSO₄ + 25g)

20g + 0.8Mass CuSO₄ = Mass CuSO₄

20g = 0.2 Mass CuSO₄

100g = Mass CuSO₄

Thus, mass of CuSO₄ you need to prepare a 80% (w/w) solution in 25mL of water is 100g