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 \(c_{t}=\sum c_{i}\) where \(M_{i}\) = molar mass, \(c_{t}\) = total concentration (\(\text{mol}\ \text{m}^{-1}\)),\(\rho \) = density of the solution (\(\text{kg}\ \text{m}^{-3}\)).