The molar heat capacity for an ideal gas at constant pressure is 20...

Molar specific heat capacity at constant pressure is defined as the amount of heat energy required to raise the temperature of one mole of a substance through $1K$ or $1 ° C$ at constant pressure. It is denoted by ${C}_{p, m}$.

Molar specific heat capacity at constant volume is defined as the amount of heat energy required to raise the temperature of one mole of a substance through $1K$ or $1 ° C$ at constant volume. It is denoted by ${C}_{v, m}$.

${C}_{v, m} = \dfrac{1}{n}\dfrac{∆ U}{∆ T}$

${C}_{p, m} = {C}_{v, m} + R$

$⇒ {C}_{v, m} = 20 . 785 - 8 . 314 = 12 . 471{Jk}^{- 1}{ml}^{- 1}$

$\Delta U = {nC}_{v, m}\Delta T$

$⇒ n = \dfrac{5000}{12 . 471 \times 200} = \dfrac{25}{12 . 471} \approx 2$