Question

16.5 mL of 2.5 M sodium sulfate is mixed with 35.5 mL of 4.5 M sodium nitrate. What is the concentration of all ions in the new solution?

Answer 1

What is your problem with this?

mols Na in Na2SO4 = 2 x (M x L) = ?
mols Na in NaNO3 = M x L = ?
mols Na in Na2SO4 + mols Na in NaNO3 = total mols Na.
Total volume = sum of the volumes.
Then M Na = mols Na/L