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Two substances A and B react with each other in such a way that A is 50% consumed in 33 minutes and 75% consumed in 66 minutes. Changing the concentration of B has no effect on the results. Which statement is true?

And the answer is:

This reaction is 1st order in A and zero order in B.

I understand why A is first order but I don't get why B is zero order. The equation for the half life of a zero order reactant is $\displaystyle\frac{[\ce{B}_\text{inital}]}{2k}$.

Doesn't this mean that the concentration for a zero order reactant would affect the half life?

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Exactly! The concentration of B during the reaction does not effect the half life. If you see for the zero order reactions, the half life can be written as $\frac{B_{\text{initial}}}{2K}$ in which B & K are constants. So for zero order reactions, half life does not depend upon the concentration of reactants at any time (after start of the reaction).

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  • $\begingroup$ But you are changing the initial concentration of B. I thought first order half lives were not affected by the initial concentration. $\endgroup$ – user99364 Jan 19 '16 at 18:04

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