How many mL of water are needed to produce 50.0mL of 1.2M CaCl2 solution from a 8.0M stock solution of CaCl2?

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?