how many moles of acid are present in 50 mL of acetic acid?

I assume we are talking about pure (=glacial) acetic acid? In this case you need to know the density, which is 1.05 g cm^-3, and the molar mass which is 60 g mole^-1

Thus the 50 ml has a mass of

1.05 g cm^-3 x 50 cm^3 = 52.5 g

hence the number of moles is

52.5 g/60 g mole^-1 =

0.875 mole