Rate of reaction question? reaction A --> products is a first order reaction. 1.60g of this reaction is allowed to react. after 38 mins the remainder is 0.4g. what is the mass after 1 hour?

Using the half-life method the answer is: 0.18g
using: (1.2/38) * 60 the answer is all of A gets used up. why is there a difference?

It is not correct to use the (1.2/38)*60 method here because it assumes that the amount changes linearly. This method would assume that in the first 38 min 1.2 g goes away and in the next 38 min. another 1.2 g goes away, etc. This obviously cannot happen.

In first-order kinetics, HALF goes away in each half-life. It is important to note that this amount changes as the reaction proceeds (0.8 g in the first half-life, 0.4 g in the next, etc.). For first-order reactions, you need to use the half-life method, not the "linear" method above.