Question

Suppose you begin with 1.50~g of the hydrate copper(II)sulfate · x-hydrate (CuSO4· x H2O), where x is an integer. After dehydration you find that you are left with 0.96~g of the an-hydrate CuSO4. What is the unknown integer x? Round your answer to the nearest integer, enter only an integer. Suppose you begin with of the hydrate KAl(SO4)2 · 12H2O. After dehydration you find that you are left with 3.0~g of the an-hydrate KAl(SO4)2. How many grams did you start with? Write the value to the correct number of significance figures.

Answer 1

1. The molar mass of CuSO4 is 159.61 g/mol. Given that there is only 0.96 g left after dehydration, the number of moles CuSO4.xH2O is,
                            m = 0.96 g / 159.61 g/mol = 6.014 x 10^-3
Thus,
                      (6.014 x 10^-3 moles)(156.61 + x(18)) = 1.50
The value of x is 5.15. For whole numbers, x is 5.

2. The number of moles of KAl(SO4) is 0.0116 moles. The molar mass of the given hydrate is 474.388 g/mol. Thus, the initial mass is 0.0116 moles x 474.388 g/mol which is equal to 5.51 g.