The weight of potassium in 0.25 moles of ${\text{KNO}}_3$  is

$\begin{array}{l}{\text{1 mole ofKNO}}_3\to 39g\text{\hspace{0.17em}of\hspace{0.17em}potassium}\\ 0.{\text{25 moles of KNO}}_3\to ? \text{g of potassium}\\ 39\times 0.25=1\times x\\ x=\frac{39\times 0.25}{1}=9.75\text{ g of potassium}\end{array}$