Question

250mL of 2.5M NH4 Cl solution is diluted down to 1.2 M. What is the new volume of solution.

Answer 1

2.5 M is the amount of substance per unit volume, i.e. liter. It means that in 0.25 mL there are

$$n=CV=2.5\cdot0.25=0.625\text{ mol}.$$

Since the substance was diluted down to 1.2 M, the new volume of the solution is

$$V_n=\frac{n}{C_n}=\frac{0.625}{1.2}=0.521\text{ L},$$

or 521 mL.