1. Assumption: in a water / HC mixture, mass fraction is the same as volume fraction.
Example 1: 99 kg HC (density 700 kg/m3) and 1 kg H2O (density 1000 kg/m3).
Mass fraction of H2O = 1/(99+1) = 1%
Vol of HC = 99/700 = m3
Vol of H20 = 1/1000 = m3
Vol fraction of H2O = (0.001)/( ) = 0.7%

2. Example 2: 70 kg HC (density 700 kg/m3) and 30 kg H2O (density 1000 kg/m3).
Mass fraction of H2O = 30/(70+30) = 30%
Vol of HC = 70/700 = 0.1 m3
Vol of H20 = 30/1000 = m3
Vol fraction of H2O = (0.03)/( ) = 23%
Assumption mass and volume fraction are the same is wrong.