Oct 12, 2014

Molarity, M, = the moles of solute/volume of solution in liters. In this problem, the solute is ethanol (EtOH) and the volume of solution is 250mL.

In order to calculate the molarity of EtOH in this solution, you must complete the following steps:

Step 1.
Determine the mass of the EtOH by multiplying its volume times its density. Density of EtOH = $\text{0.789g"/"mL}$

mass = volume x density = $\text{75mL}$ x $\text{0.789g"/"mL}$ = $\text{59g}$ EtOH

Step 2.
Calculate the moles of EtOH by dividing its mass by its molar mass. Molar mass of EtOH = $\text{46.068g/mol}$

moles EtOH = mass/molar mass = $\text{59g"/"46.068g/mol}$ = $\text{1.3mol}$ EtOH

Step 3.
Calculate molarity, M, by dividing the moles of EtOH by liters of solution. Liters of solution = $\text{250mL}$ x $\text{1L"/"1000mL}$ = $\text{0.25L}$

M EtOH = mol EtOH/L = $\text{1.3mol EtOH"/"0.25L}$ = $\text{5.2M}$

The molarity of ethanol in this solution is $\text{5.2M}$.