R = 8.314;
P = 2.494*(10^6);
CA[XA_, 
          T_] = ((P*(1 - XA))/(R*T));
CR[XA_, T_] = ((P*XA)/(R*T));
k1[
          T_] = (E^(7 - (83700/(R*T))));
k2[
          T_] = (E^(18 - (167400/(R*T))));
rA[XA_, T_] = k1[T]*CA[XA, T] - k2[T]*CR[XA, T]

sol = Solve[rA[XA, T] == rA, XA];
XA[T_, rA_] = XA /. sol[[1]];

\(\(fig = 
        Plot[{XA[T,  .0000001], XA[T, 0.000001], XA[T, 0.00001], 
            XA[T, 0.00002], XA[T, 0.00005], XA[T, 0.0001], XA[T, 0.0002], 
            XA[T, 0.0005], XA[T, 0.001], XA[T, 0.002], XA[T, 0.005], 
            XA[T, 0.01], XA[T, 0.02], XA[T, 0.05], XA[T, 0.1], XA[T, 0.2], 
            XA[T, 0.3], XA[T, 0.4], XA[T, 0.5], XA[T, 0.6], XA[T, 0.8], 
            XA[T, 1], XA[T, 1.3], XA[T, 1.5], XA[T, 1.8], XA[T, 2], 
            XA[T, 2.3], XA[T, 2.6], XA[T, 3], XA[T, 3.5], XA[T, 4]}, {T, 300, 
            900}, PlotRange -> {{300, 900}, {0, 1.05}}, 
          AxesOrigin -> {300, 0}, Frame -> True, 
          FrameLabel -> {"Temperature, K", "Conversion"}, 
          GridLines -> {{325, 350, 375, 400, 425, 450, 475, 500, 525, 
                550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 
                850, 875}, { .05,  .1,  .15,  .2,  .25,  .3,  .35,  .4,  .45,  .5,  .55,  .6,  .65,  .7,  .75,  .8,  .85,  .9,  .95, 1}}, 
          FrameTicks -> {{300, 350, 400, 450, 500, 550, 600, 650, 700, 
                750, 800, 850, 
                900}, {0,  .1,  .2,  .3,  .4,  .5,  .6,  .7,  .8,  .9, 
                1}, {}, {}}];\)\)