An oxygen molecule of mass 5.3 × 10⁻²⁶ kg moves along x at 480 m/s between two walls 0.10 m apart…

Step 1. Time between hits. $t = \dfrac{2L}{v_x} = \dfrac{2 \times 0.10\ \text{m}}{480\ \text{m/s}} = 4.17 \times 10^{-4}$ s.

Step 2. Hits each second. $\dfrac{1}{t} = \dfrac{480\ \text{m/s}}{0.20\ \text{m}} = 2400$ hits each second.

Step 3. Momentum per hit. $2mv_x = 2 \times 5.3 \times 10^{-26}\ \text{kg} \times 480\ \text{m/s} = 5.09 \times 10^{-23}$ kg m/s.

Step 4. Force. $F = 5.09 \times 10^{-23}\ \text{kg m/s} \times 2400\ \text{s}^{-1} = 1.22 \times 10^{-19}$ N.