Ethyl chloride, CH3CH2Cl, used to produce tetraethyllead gasoline additive, decomposes, when heated, to give ethylene and hydrogen chloride.

CH3CH2Cl(g) †’ C2H4(g) + HCl(g)
The reaction is first order. In an experiment, the initial concentration of ethyl chloride was 0.00100 M. After heating at 500oC for 155 s, this was reduced to 0.00067 M. What was the concentration of ethyl chloride after a total of 256 s?