[kmhard1]

Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0.  He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be.  Is he correct?

[Borek]

In general, forcing anything through zero is a bad idea. There are zillions of possible reasons that can shift your otherwise perfectly straight line in some direction, so that it doesn't go through 0.

[Hunt]

Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0.  He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be.  Is he correct?

When you plot Ln([A]) = f(t) , you'll get a straight line with an intercept equal to Ln[A]₀ , which is a constant ofcourse and not a variable.

The relation ln-([A] - [A]o) =f(t) doesnt work for a 1st-order reaction.

[tamim83]

ln[A]=ln[A₀]-kt should though.  k is the rate constant.  So plotting ln[A] vs. t gives you the rate constant (-k=slope) and ln[A₀] (the y-intercept).