0.242 g sample of potassium is heated in oxygen. The result is 0.440 g of a crystalline compound. What is the formula of this compound ?

Let the chemical reaction of K and oxygen is:

$\mathrm{xK}+{\mathrm{yO}}_2\to \mathrm{product}$

Mass of sample of K is 0.242 g and the mass of product forms is 0.440 g. Mass of oxygen gas is:

$\begin{array}{ccc}\mathrm{Mass} \mathrm{of} \mathrm{oxygen}& =& 0.440 \mathrm{g}-0.242 \mathrm{g}\\ & =& 0.198 \mathrm{g}\end{array}$

Molecular mass of O₂ is 32 g. Moles of oxygen utilized by O₂ is:

$\begin{array}{ccc}\mathrm{Moles} \mathrm{of} {\mathrm{O}}_2& =& \frac{0.198 \mathrm{g}}{32 \mathrm{g}/\mathrm{mol}}\\ & =& 6.2\times {10}^{-3} \mathrm{mol}\end{array}$

Moles of K in 0.242 g of K.

$\begin{array}{ccc}\mathrm{Moles} \mathrm{of} \mathrm{K}& =& \frac{0.242 \mathrm{g}}{39.1 \mathrm{g}/\mathrm{mol}}\\ & =& 6.2\times {10}^{-3} \mathrm{mol}\end{array}$

As mole ratio of K and O₂ is 1:1, hence the product formed is KO₂.