Verified answer

From ideal gas law

P∙V = n∙R∙T

follows

P/T = n∙R/V constant

because neither the amount of gas enclosed in the flask nor the flask volume changes.

Actually the will be a slight change of the volume due to thermal expansion of the flask, but you can ignore that effect for this problem

Hence,

P_final/T_final = P_initial/T:_initial

Thus the temperature at which the gas in the flask has reached a certain pressure is given by:#

T_final = T_initial ∙ (P_final / P_initial)

Initial pressure is not given. So let's assume that the flask was filled with gas at standard atmospheric pressure, i.e.

P_initial = 1 atm = 760 torr

Then the gas will reach the critical pressure at a temperature of

T_final = (21 + 273) K ∙ (2508 torr / 760 torr )

= (21 + 273) K ∙ (2508 torr / 760 torr )

= 970 K

= 697 °C