If 240 mL of water is added to 13 mL of 5.0 M NaCl solution,

what is the concentration of the dilute solution?

The correct answer is 0.26 M

Explanation:

The initial concentration of NaCl solution (Ci) is 5.0 M. The initial volume of the solution (Vi) is 13 mL. The final volume is the addition of 240 mL to 13 mL (Vf = 13 mL + 240 mL = 253 mL).

In order to calculate the final concentration of the solution (Cf) we can use the following expression:

Ci x Vi = Cf x Vf

This mathematical expression is often used to calculate concentrations in dilution procedures. So, we introduce Ci, Vi an Vf in the expression and we calculate Cf:

Cf = (Ci x Vi) / Vf = (5.0 M x 13 mL) / 253 mL = 0.257 M ≅ 0.26 M