[Rutherford]

A test tube sealed with a rubber stopper contains equal volumes of chlorine and hydrogen gases at 20 C and 101 kPa. When the test tube is exposed to bright sunlight, the stopper is shot out. Calculate the degree of conversion of the initial gases at the instant the stopper is shot out. Assume that the gases are ideal and have the same heat capacity of 20 J/mol K. The standard enthalpy of formation of hydrogen chloride is -92.3 kJ/mol. The stopper is shot out when the pressure inside the test tube exceeds the atmospheric pressure by 15%.

1/2H₂ + 1/2Cl₂ HCl+92.3kJ
n-1/2x    n-1/2x      x
The pressure rises because of the temperature increase (V=const.)
p₁/p₂=T₁/T₂
ΔT=43.95K
Q=mcΔT=ΔH*x
m=(1/2*2+1/2*71)n=36.5n
x=0.35n or 35%, while the answer is 0.953%, way too different. Where is the mistake?

[Borek]

What do you use masses for, if all the data refers to moles?

[Rutherford]

Yeah, it should be 2n instead of m in the formula. The amount that reacted is 1/2x and I got the right answer. Thanks.