There\'s a proportionality constant (the rate constant) and the effect of concentration in a rate-law expression. Since the concentration of CO has no effect, I\'ll leave it out. Rate = - d[NO2]/dt = k[NO2]^2 Solution There\'s a proportionality constant (the rate constant) and the effect of concentration in a rate-law expression. Since the concentration of CO has no effect, I\'ll leave it out. Rate = - d[NO2]/dt = k[NO2]^2.