The volume of 3 mol of hydrogen in a vessel at a temperature of 300 K and pressure P1 is equal to V1.

The volume of 3 mol of hydrogen in a vessel at a temperature of 300 K and pressure P1 is equal to V1. What is the volume of 3 moles of oxygen in a vessel at the same temperature and the same pressure?

Let’s write the Mendeleev-Clapeyron equation for both gases:
p * v = V1 * R * T1. (V1, V2 is the number of moles).
p * v = V2 * R * T2. The left parts are the same, we equate the right ones and reduce by R.
V1 * T1 = V2 * T2.
T2 = T1 * V1 / V2.