Can someone please help me with this chemistry question?

Question

Given that the value of Ksp of CuSO4.5H2O at 60 degrees Celcius is 1.6 mol/L. Now, 100mL of 2.0 M Cu(NO3)2 solution at 60 C had small amounts of Na2SO4 placed into it with thorough stirring after each addition. How many moles of Na2SO4 must be added before crystals of CuSO4 are observed that do not dissolve with stirring?

A. 0.110 mol B. 0.128 mol C. 0.129 mol D. 1.28 mol

Answer 1

Find the Ksp, if the molar solubility is 1.6 mol/litre CuSO4, then the

Ksp = [Cu+2] [SO4-2]

Ksp =  [1.6] [1.6]

Ksp = 2.56

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now use the Ksp to find the SO4-2 that can stand to be with 2.0 Molar Cu+2:

Ksp = [Cu+2] [SO4-2]

2.56 = [2.0M] [SO4-2]

SO4 = 1.28 moles / litre

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if there can be 1.28 moles of sulfate in one liter, find the amount in 100 ml:

SO4 = 0.128 moles in 100 ml

your answer is : B 0.128 moles