To calculate the mass of copper(II) sulfate pentahydrate needed to obtain 10.0 g of anhydrous copper(II) sulfate, we need to consider the difference in their molecular weights.

- Copper(II) sulfate pentahydrate has the formula CuSO4.5H2O and a molar mass of approximately 249.69 g/mol.

- Anhydrous copper(II) sulfate has the formula CuSO4 and a molar mass of approximately 159.61 g/mol.

The conversion can be set up as a proportion based on the molar ratios of the compounds:

(Mass of CuSO4.5H2O) / (Molar mass of CuSO4.5H2O) = (Mass of CuSO4) / (Molar mass of CuSO4)

Rearranging the equation to solve for the mass of CuSO4.5H2O:

(Mass of CuSO4.5H2O) = (Mass of CuSO4) × (Molar mass of CuSO4.5H2O) / (Molar mass of CuSO4)

Plugging in the values:

(Mass of CuSO4.5H2O) = 10.0 g × (249.69 g/mol) / (159.61 g/mol)

Calculating the result:

(Mass of CuSO4.5H2O) ≈ 15.63 g

Therefore, you would need approximately 15.63 grams of copper(II) sulfate pentahydrate to obtain 10.0 grams of anhydrous copper(II) sulfate.