Solving Concentration Problems: Sodium Cations

[Kyoma]

Homework Statement

15 cm³ of 0.200 mol dm^-3 sodium hydroxide solution was made up to 100 cm³ with 0.120 mol dm^-3 sodium sulfate solution. Determine the molar concentration of sodium cations in the resultant mixture.


2. The attempt at a solution
Moles of sodium ions in sodium hydroxide = 0.200 x (15/1000) = 0.003 mol
Let volume of sodium sulfate be V_x.
(0.120V_x) x 2 + 0.003 = Total moles of sodium ions

After which, i find it hard to proceed. :(

 

[Borek]

You started with 15 mL, now you have 100 mL. How many mL were added?

 

[Kyoma]

85 mL? But I remember that particles can fit into small spaces, so surely 85 mL is wrong?