You prepare a 2nd solution (solution B) by pipetting 10.00 ml of your 1st solution (solution A) into a 50.00 ml volumetric flask and diluting with deionized water to the 50.00 ml mark.
If the concentration of solution B is 0.1045 M NaCl, what was the NaCl concentration of solution A?

Concentration solution A was 0.5225 M

Explanation:

10.00 mL of solution A was diluted to 50.00 mL and yields 50.00 mL of solution B

According to laws of dilution-      

where, and are concentration of solution A and B respectively

and are volumes of solution A and B respectively

Here = 0.1045 M, = 50.00 mL and = 10.00 mL

Hence,

So, concentration solution A was 0.5225 M