[mollyp]

I cannot for the life of me figure this out:

What mass of Na₂SO₄ is needed to prepare 350.0 mL of a solution having a sodium ion concentration of 0.125M?

I know that there are 2 moles of Na for every 1 mole of SO₄. 

Now do I multiply that by 3, since there are 2 moles of Na & 1 mole of SO4?

I don't know when to use my volume. 
Argggg.. any suggestions?
Thanks

[AWK]

I know that there are 2 moles of Na for every 1 mole of SO4.

Then molarity of Na₂SO₄ is....

Having known molarity of sodium sulfate you can find moles of this salt in 350,0 ml then mass of sodium sulfate

[mollyp]

Ok, I got it!!
350mL x 1L/1000mL x 0.125mols/1L = .0438 mols

.0438mols/2 mols = .0219 mols

.0219mols x 142g/1 mol = 3.11 g Na₂SO₄

That only took me about 3 hours....ugggg! 

[mollyp]

THANK YOU!!!!!!!!!!!!!!!