The temperature of an ideal gas is raised from 10°C to 20°C while the volume is kept constant. If the pressure at 10°C is P_i, what is P_f, the pressure at 20°C?

1. P_f < P_i
2. P_f = P_i
3. P < P_f < 2P_i
4. P_f = 2P_i
5. P_f > 2P_i