Textbook example 8.6

[Gurvardaan Bal1L]

For part b on example 8.6(on page 281) why is delta U equal to q using the heat capacity at constant volume? I thought that at constant pressure the change in internal energy is equal to the change in enthalpy, so I'm confused as to why we are measuring q using the heat capacity at constant pressure. I understand that the book says that the gas must be heated at constant pressure to the final temperature, but why?

[Lucian1F]

Since the system is at a constant volume, the work is 0, so the change in internal energy is equal to heat, so delta U = q.