A solution is made using 200.0 mL of methanol (density 0.7920 g/mL) and 789.1 mL of water (density 1.000 g/mL). What is the mass percent of the methanol?

Answer 1

To find the mass % of methanol we need the mass of methanol, the mass of water and the total mass of solution.

Mass of methanol = 200.0 ml x 0.7920 g/ml = 158.4 g

Mass of water = 789.1 ml x 1 g/ml = 789.1 g

Total mass of soln = 158.4 g + 789.1 g = 947.5 g

Mass % methanol = 158.4 g/947.5 g (x100) = 16.72%

Answer 2

% by mass of methanol = m_M/(m_M+m_W) * 100

m_{M =} ρ_MV_M

m_W= ρ_WV_W

All the units mesh, so it is now just arithmetic.