How many grams of cuso4 5h20 are needed to prepare a 0.020 m stock solution?

Molar mass of CuSO4*5H2O = 63.5 + 32 + 4(16) + 5(18) = 249.5 g/mole 

molarity is calculated as follows:
molarity = number of moles of solutes/liters of solution

We will assume that the number of liters = 1
Substituting in the above equation:
0.02 = number of moles of CuSO4*5H2O / 1
number of moles = 0.02 moles

number of moles = mass/molar mass
mass = number of moles * molar mass
mass = 0.02 * 249.5 = 4.99 grams