Speed of gas in the nozzle
(if exhaust speed is obtained by warming)

1) first method
Maximum speed = ((G*R*T/M)^0,5)*(2/(G-1))^0,5
Speed of sound = (G*P/M)^0,5

Example with hydrogen :
1,45 G is the ratio of specific heat at constant pressure to specific heat at constant volume
0,002 M is the molecular mass
P is the pressure

Gas speed in the nozzle : 5175 m/sec
ISP 528

2) Another method (in Systèmes Spatiaux by D Marty)

Maximum speed = SquareRoot((2*G/(G-1))*(R/M)*T*(1-(1/((internal pressure/external pressure)^((G-1)/G)))))

Gas speed in the nozzle : 4760m/sec

ISP 486

3) some materials

Ratio of specific heats

Molecular mass

Dry air

1,39

0,0288

Hydrogen

1,45

0,002

Methane

1.3

0.016

Oxygen

1.41

0.032

Steam

1.3

0.034

H2O

1.43

0.034

CO2

1.355

0.048