Calculate the molarity of a solution containing 85.0 g of sodium sulfate, Na, SO4, in 325 mL of solution?

Can you guys me with question no. 5!

What is a metal replacement reaction

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?