Formula from Chapter 9: chemical amount of a gas component
\(P_{\mathrm{component}} \cdot V_{\mathrm{gas}} = n_{\mathrm{component}} \cdot R \cdot T\)
You can use this if: the sample is a gas mixture such as air
\(R\) \(= 8.314\ \frac{\mathrm{J}}{\mathrm{mol}\ \mathrm{K}}\)
\(T\) \(= 298.15\ \mathrm{K}\) room temperature
\(P_{\mathrm{\ce{O2}}}\) \(= 0.19\ \mathrm{atm}\) partial pressure of oxygen at sea level
\(V_{\mathrm{gas}}\) \(= 25.0\ \mathrm{L}\)
\(n_{\mathrm{\ce{O2}}}\) \(= \dfrac{P_{\mathrm{\ce{O2}}} \cdot V_{\mathrm{gas}}}{R \cdot T}\)
\(\ \ \ =\dfrac{0.19\ \mathrm{atm} \cdot 25.0\ \mathrm{L}}{8.314\ \frac{\mathrm{J}}{\mathrm{mol}\ \mathrm{K}} \cdot 298.15\ \mathrm{K}}\)
\(\ \ \ =\dfrac{4.75\ \mathrm{atm}\ \mathrm{L}}{2478.82\ \frac{\mathrm{J}}{\mathrm{mol}}}\)
\(\ \ \ =0.19\ \mathrm{mol}\)