a sample of copper sulphate pentahyrdrate contain 3.2g Cu. How many grams of oxygen are there in this sample?

 copper sulphate pentahyrdrate = CuSO4.(H₂O)5

CuSO4.(H2O)5 -------------->    CuSO4  +  5H₂O

moles of copper = 3.2/63.5 = 0.05

.i.e. moles of CuSO4.(H₂O)5 = 0.05

moles of oxygen = 0.05 x 9 = 0.45

mass of oxygen in ''gm'' = 0.45 x 16 = 7.2 gm

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