A solution prepared by dissolving 20.2 mL of CH3OH in 100mL of water @ 25 celcius. The final volume of the solution is 118 mL. Calculate the % by mass.

density CH3OH[0.782g/mL], H20[1g/mL] @ room temp.

%w/w = mass solute/100 g soln

mass solute = 20.2 mL x 0.782 g/mL = ?
mass soln = 100 g(H2O) + ?g solute = xx
%by mass = (?g solute/xxg soln)*100 =z%

To calculate the percent by mass of CH3OH in the solution, we first need to determine the mass of CH3OH and the mass of the solution.

1. Calculate the mass of CH3OH:
The density of CH3OH is given as 0.782 g/mL. Since we have 20.2 mL of CH3OH, we can use the density to calculate its mass.
Mass of CH3OH = volume of CH3OH x density of CH3OH
Mass of CH3OH = 20.2 mL x 0.782 g/mL
Mass of CH3OH = 15.7864 g

2. Calculate the mass of the solution:
The mass of the solution can be calculated by adding the mass of CH3OH to the mass of water used.
Mass of Water = volume of water x density of water
Mass of Water = 100 mL x 1 g/mL
Mass of Water = 100 g
Mass of Solution = Mass of CH3OH + Mass of Water
Mass of Solution = 15.7864 g + 100 g
Mass of Solution = 115.7864 g

3. Calculate the percent by mass of CH3OH in the solution:
Percent by mass = (mass of CH3OH / mass of solution) x 100
Percent by mass = (15.7864 g / 115.7864 g) x 100
Percent by mass ≈ 13.64%

Therefore, the percent by mass of CH3OH in the solution is approximately 13.64%.