Exactly 1.5 g of a fuel burns under conditions of constant pressure and then, in a
second experiment, under conditions of constant volume. in one case (not
necessarily the first), the reaction produces 25.9 kj of heat and in the other it
produces 23.3 kj of heat. which measurement corresponds to the conditions of
constant pressure? justify your answer.

At a constant pressure the production of heat will be 25.9 KJ.

Explanation:

The heat produces at constant pressure will be more as compared to the heat produced at constant volume because,

At a constant pressure there is some work done and heat is used in this process. All the heat is not going to change the temperature. while,

At constant volume there is no work done and all the heat is going to change the temperature. In other words we can say that it take less heat to produce the temperature change at  a constant volume as compared to at constant pressure.

Cv ∠ Cp

So, when 1.5 g of fuel is burn at constant pressure it will produce 25.9 KJ. while at constant volume it will produce 23.3 KJ.