A solution is prepared by dissolving 20.2 mL of methanol (CH₃OH) in 100.0 mL of water at 25 °C. The final volume of the solution is 118 mL. The densities of methanol and water at this temperature are 0.782 g/mL and 1.00 g/mL, respectively. For this solution, calculate the concentration in each unit. a. molarity

Key Concepts

Molarity

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. It is expressed in moles per liter (mol/L). To calculate molarity, one must first determine the number of moles of the solute, which can be found using the formula: moles = mass (g) / molar mass (g/mol). The final volume of the solution must be in liters to use in the molarity formula.

Density

Density is defined as mass per unit volume and is typically expressed in grams per milliliter (g/mL) for liquids. It is crucial for converting between mass and volume, especially when calculating the mass of a solute from its volume and density. In this problem, the densities of methanol and water are used to find the mass of methanol that is dissolved in the solution.

Final Volume of Solution

The final volume of a solution is the total volume after the solute has been added to the solvent. It is important to note that the final volume may not equal the sum of the individual volumes of solute and solvent due to volume contraction or expansion upon mixing. In this case, the final volume is given as 118 mL, which is necessary for calculating the molarity of the solution.