The half-life of cobalt-69 is 6 years. How much of a 5-gram sample is left after 12 years

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?