Can someone explain me how to get the answer of this question, please?

Can you guys me with question no. 5!

How many miles of water are produced if 5.43 mol pbo2 are consumed

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?