Since we know that 1.245g of Cu(OH)2 was obtained after the solid precipitated in water, it means that the remaining has to be CuSO4. Thus, we can take the difference between the two masses to get the mass for CuSO4.
After that, divide the mass of CuSO4 by the overall mass of the impure substance, multiply it by 100, and you will get the percentage of purity.
