To determine k

[Ekayana Sethi 2K]

2 A --> B + C, given that [A] is 0.153 mol!L"1 and that after 115 s the concentration of B rises to 0.034 mol!L"1.
In this we find At by subtracting 0.068 from Ao. but if B was a reactant and it was increasing would we add it instead of subtracting?

[Sarah_Stay_1D]

2 A --> B + C, given that [A] is 0.153 mol!L"1 and that after 115 s the concentration of B rises to 0.034 mol!L"1.
In this we find At by subtracting 0.068 from Ao. but if B was a reactant and it was increasing would we add it instead of subtracting?

The reason that they are subtracting in this case is because the concentration of the reactants is decreasing. You are right in that the concentration of B will rise during the reaction because it is a product. However, if B were a reactant, it's concentration would be decreasing like A, not increasing. Therefore, if B were a reactant, and you were given the change in concentration for a product, you would subtract the stoichiometrically correct number of moles/L from [B]o.

[Jeremiah Samaniego 2C]

Based on the chemical equation, A is a reactant and B is a product. If product B is being formed, then it must mean that reaction A is being consumed. We want to find the equivalent of reactant A being consumed in relation to the product B. By finding the molar ratio between A and B, we can see that for every mole of B produced, 2 moles of A are used up. In other words, if 0.034 mol L^-1 of B are being formed, 2 (0.034 mol L^-1 ) = 0.068 mol L^-1 of A will be used up, which is why we subtract 0.068 mol L^-1 from the initial concentration of A.