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