t = [0:10:180]; %waktu reaksi
Xa = [1,0.9783,0.9565,0.9239,0.9022,0.8587,0.8370,0.7826,0.7500,0.7174...,
      0.6957,0.6739,0.6522,0.6304,0.5978,0.5435,0.5435,0.5435,0.5435]; %(1-Xa)
y = -log (Xa); %-ln(1-Xa)
x = t;
[s] = polyfit (x,y,1);
k = s(1) %konstanta kecepatan reaksi