why does methanol give higher back pressure than acenitrile in HPLC analysis?

Answer 1

Although the densities are similar (0.7918 g/cm3 for Methanol and 0.786 g/cm3 for Acetonitrile), The viscosities are very much different.

The viscosity of Methanol is 0.59 cP at 20° C. The viscosity of Acetonitrile is 0.38 cP at 15° C and 0.345 cP at 25° C, or about 60% of that of Methanol.

It is this higher viscosity that gives the liquid the higher back pressure.