So what do I do after I divide the mass by 2? Refer to previous question please
You would have done well to paste your question here so we didn't need to look through all of the other questions to find yours.
You want 250 mL of 0.34M Na^+ and you want to use Na2SO4.10H2O.
How many moles Na^+ do you need? That will be M x L = 0.250L x 0.34M = 0.085 moles. There are two Na^+ in 1 molecule of Na2SO4.10H2O; therefore, we will need 0.085/2 moles of the decahydrate or 0.0425. The molar mass of Na2SO4.10H2O is 322.2 so grams = moles x molar mass or about 13.7 grams (not exact).