how many grams of nitrogen are in 50.0 g of caffeine (C8H10N4O2) explain how you got your answer please

Can someone me with questions 12 and 13 plz.

|using the periodic tablewarm-upuse the periodic table in the tools bar to answer the following questions. what elemental classification does oxygen belongto? done

Abeaker with 2.00×102 ml of an acetic acid buffer with a ph of 5.000 is sitting on a benchtop. the total molarity of acid and conjugate base in this buffer is 0.100 m. a student adds 4.70 ml of a 0.360 m hcl solution to the beaker. how much will the ph change? the pka of acetic acid is 4.740.