How to find moles given oxidation numbers and chemical formula? Trying to do my

Question

How to find moles given oxidation numbers and chemical formula? Trying to do my chemistry lab but i'm stuck. this is the part i'm stuck on:

The overall charge on the complex, [Ni(en)(H2O)]^2+ * SO4^2- * H2O is zero; the charge on sulfate ion is 2-;en and water ligands both have a charge of zero; and nickel has a charge of 2+. Using this information and your previous experimental results, calculate the number of moles of SO4^2- in 100 grams of your complex and the mass percent of SO4^2- in your complex.

previous experimental results: Molarity of Ni^2+ in unknown sample: 0.036M.

Moles of Ni^2+ in 100 g of complex: 0.345 moles

Mass % of Ni^2+ in complex: 20.24%

Explanation / Answer

Solution:- from the given complex formula, there is 1:1 mole ratio between Ni+2 and SO4-2. there are 0.345 moles of Ni+2, So, the moles of SO4-2 are also 0.345.

molar mass of SO4-2 is (32 + 4*16 = 96 g/mol)

So, 0.345 mol x (96g/1mol) = 33.12g

mass percent od SO4-2 = (mass of SO4-2/mass of complex) x 100 = (33.12/100) x 100 = 33.12%

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