A 15 litre cylinder holds oxygen at 290 K and 4.0 × 10⁵ Pa.

Step 1. Moles. $\dfrac{4.0 \times 10^{5}\ \text{Pa} \times 0.015\ \text{m}^3}{8.31\ \text{J mol}^{-1}\text{K}^{-1} \times 290\ \text{K}} = 2.49\ \text{mol}$.

Step 2. Molecules. $2.49\ \text{mol} \times 6.02 \times 10^{23}\ \text{mol}^{-1} = 1.50 \times 10^{24}$.

Step 3. Mass. $2.49\ \text{mol} \times 0.032\ \text{kg/mol} = 0.0797\ \text{kg}$, about $80\ \text{g}$.