How many valence electrons does a nitrogen atom have?

Can someone me with questions 12 and 13 plz.

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?

Which of the following units is not an official si unit