A --> B + C --> A (k1,k2)
y[t] = [A](t)
y’ = -k1 y + k2 (A0 - ((y))^2)

eqn=y'[t]==-k1 y[t]+k2 (A0-y[t])^2

A0=1; k1=1;k2=2;
sol=DSolve[{eqn,y[0]==1},y,t]

yf=sol[[1]]

CA[t_]:=y[t] /. yf

CB[t_] :=A0 - CA[t]

Plot[{CA[t] ,CB[t]}, {t,0,5},AxesLabel->{time,Concentration},PlotRange->{{0,3},{0,1}},PlotLegends->"Expressions"]

eqnx=x^2/(1-x)==k1/k2

Solve[eqnx,x]

N[%]

CA[0.5]