Density and speed of sound have been measured earlier for binary liquid mixtures formed by formamide, N-methylacetamide, di-methylformamide and di-methylacetamide with acetonitrile at 293.15, 298.15, 303.15, 308.15 and 313.15 K and atmospheric pressure over the whole concentration range. Pseudo-Gruneisen parameter and internal pressure have been derived from the measured values of density and ultrasonic velocity. These values are compared with the theoretical values obtained by the utilization of Flory theory, Ramaswamy and Anbananthan model and model suggested by Glinski to predict the behaviour of weakly interacting liquids. The observed properties derived from measured data are fitted to Redlich-Kister polynomial relation to estimate the binary coefficients and standard errors. Excess internal pressure parameter for these binary mixtures is also computed to study the molecular interactions involved in the liquid systems.