3)
0.09 moles of sodium sulfate in 12 ml of solution.

7.5 M

Explanation:

In order to find a solution's molar concentration, or molarity, you need to determine how many moles of solute, which in your case is sodium sulfate,  

Na

2

SO

4

, you get in one liter of solution.

That is how molarity was defined -- the number of moles of solute in one liter of solution.

So, you know that you have  

0.090

moles of solute in  

12 mL

of solution. Your goal here will be to scale up this solution by using this information as a conversion factor to help you determine the number of moles of solute present in

i do not get it   at all

c) should be the answer.