Density = 12 g/mL and the Volume = 4 mL. What is the mass?

Idon’t understand this page, can someone ?

What is the number of moles of chemical units represented by 9.03x10^24? and how do i show work? (dumb it down )

If you add 5.00 ml of 0.100 m sodium hydroxide to 50.0 ml of acetate buffer that is 0.100 m in both acetic acid and sodium acetate, what is the ph of the resulting solution? acetic acid: ka = 1.8. x 10-5