Mass-average velocity: \[\nu_{b} = \rho ^{-1}\ \sum C_{i}M_{i}\nu_{i}\] Cf. molar average velocity: \[\nu_{m} = c_{t}^{-1}\ \sum c_{i}\nu_{i}\] with ct = ∑ci where M i = molar mass, ct = total concentration (mol m-1),ρ = density of the solution (kg m-3).