If i have 40 cigarettes And 12 plants to test them on for 3 weeks, how many cigarettes should I put and how many days till i should put another?

Imagine that twenty i.u.’s of enzyme z were catalyzing the above reaction for one minute, under vmaxconditions, in a 3.00 ml assay volume. the assay is buffered with 20 mm phosphate buffer, ph 7.60. what will the ph be at the end of that one minute?