The molarity M of a solution is the quotient between the amount of substance n and the volume V:
[tex]M=\frac{n}{V}[/tex]Substitute n=5.25 mol and V=300 mL = 0.3 L to find the molarity:
[tex]M=\frac{5.25\text{ mol}}{0.3\text{ L}}=17.5\text{ mol/L}[/tex]