[jayjay112]

2. How much of each of the following must be dissolved in water to produce 250 cm3 of a solution that is 0.1 M in Na+ ion?
(i) Na2SO4 ; (ii) Na2SO4.10H2O (iii) Na3PO4

(i) (250/1000) x 0.1 = 0.025
0.025 x molar mass = 0.025 x 142 = 3.55g but the answer is half (1.775g)

(ii) (250/1000) x 0.1 = 0.025
0.025 x molar mass = 0.025 x 322 = 8.05g but the answer is half (4.025g)

(iii) (250/1000) x 0.1 = 0.025
0.025 x molar mass = 0.025 x 164 = 4.1g but the answer is a third of this (1.367g)

do you know what i am doing wrong??
Thanks

[ARGOS++]

Dear jayjay112;

You simply forgot, that in every of your substances are more then only one Na⁺ contained!

Good Luck!
                    ARGOS⁺⁺

[jayjay112]

so what does that mean when i am calculating molar mass? divide your mass by the mulitple of the Na??

[ARGOS++]

Dear jayjay112;

That means that if you have 5 Na⁺ in your formula, then you need only one-fifth of a mole for the same amount of Na⁺.

Good Luck!
                    ARGOS⁺⁺

[enahs]

What happens when it dissolves?

Na₂SO₄  2 Na⁺ + SO₄^-

So if one mol of Na₂SO₄ gives you 2 mols in solution of Na⁺, then how many mols of Na₂SO₄ do you need to get 1 mol of Na⁺?

Chemical equations for the win!