Chemistry salt crystals question

Q) 3.715g of crystals heated until no further loss in mass. Anhydrous salt had a mass of 2.086g.

ZnSO₄.xH₂O(s) ---> ZnSO₄ (s) + xH₂O(g)

a)
i) How many moles of zinc sulphate are there in 2.086g of anhydrous zinc sulphate ?

Mass of anhydrous ZnSO₄ = 2.086g
Mr of ZnSO₄= 65 + 32 + (16x4) =161 g/mol

So:
Moles of ZnSO₄= Mass/Mr =2.086g / 161g mol = 0.0129 mol

ii) How many moles of water were lost?

Hydrated ZnSO₄– Anhydrous ZnSO₄= Mass of water
3.715g – 2.086g = 1.629 g
Mr of H₂O = 18 g/mol
Moles = Mass/Mr = 1.629g ÷18g/mol = 0.0905 mol

iii) What is the value of x in ZnSO₄.xH₂O ?
So ZnSO₄: H₂O
0.0129/0.0129 : 0.0905/0.0129
∴ ZnSO₄.7H₂O

b) Daily recommended dose of zinc in USA is 15 mg

i) What mass of zinc sulphate crystals would need to be taken to obtain this intake?

ii) If this is taken via a 5 cm³ dose of aqueous zinc sulphate calculate the concentration of the solution in mol dm^-3of the hydrated salt?

Can someone help with part b) ?

(edited 8 years ago)

Reply 1

Infraspecies

b.(i)

Calculate the moles of Zn in 15 mg.

Calculate the mass of that number of moles of hydrated zinc sulfate crystals.

b.(ii)

n=cV

You know n and V.

(edited 8 years ago)

Reply 2

M.Abubaker

Original post by Infraspecies

b.(i)

Calculate the moles of Zn in 15 mg.

Calculate the mass of that number of moles of hydrated zinc sulfate crystals.

b.(ii)

n=cV

You know n and V.

Apparently the answer is still wrong

Calculates moles of zinc in 15mg =
Moles = mass / mr = 0.015 g / 65 g/mol = 0.0002 mol

Mass of hydrated zinc sulphate = moles x mr
Mr of ZnSO4.7H2O = 191 g/mol
= 0.0002 mol x 191 g/mol = 0.044 g = 44 g