gasConst = 8.314;
kRatio = 
        1.9  Exp[-15000/gasConst/temp]/(30 
                 Exp[-20000/gasConst/
                    temp]);
Plot[kRatio, {temp, 278, 363}, 
        PlotRange -> {0. , 0.55}];

kRatio /. temp -> 90 + 273

tempOpt = 90 + 273;
k2 = 
        1.9  Exp[-15000/gasConst/
              tempOpt];
k1 = 
        30  Exp[-20000/gasConst/
              tempOpt];
tOpt = Log[k2/k1]/(k2 - k1) // N

cRnorm3 = 
        k1/(3  k1 - k1)*(Exp[-k1Tau] - 
              Exp[-3 k1Tau]);
cRnorm2 = 
        k1/(2  k1 - k1)*(Exp[-k1Tau] - 
              Exp[-2 k1Tau]);
cRnorm05 = 
        k1/(0.5  k1 - k1)*(Exp[-k1Tau] - 
              Exp[-0.5 k1Tau]);
cRnorm03 = 
        k1/(0.3  k1 - k1)*(Exp[-k1Tau] - 
              Exp[-0.3 k1Tau]);
cRnorm01 = 
        k1/(0.1  k1 - k1)*(Exp[-k1Tau] - 
              Exp[-0.1 k1Tau]);
cRnorm0 = 1 - Exp[-k1Tau];

Plot[{cRnorm3, cRnorm2, cRnorm05, cRnorm03, cRnorm01, 
          cRnorm0}, {k1Tau, 0, 5}, 
        PlotRange -> {0. , 0.95}];

cRmaxOverCA0 = (1/kRatio /. 
            temp -> 
              90 + 273)^(1/(1 - 1/kRatio /. 
                temp -> 90 + 273))