Ch.14 - Solutions

Problem 70b

Previous problem

Next problem

Chapter 14, Problem 70b

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. b. molality

Verified step by step guidance

Calculate the mass of methanol using its volume and density. Use the formula: mass = volume × density.

Calculate the mass of water using its volume and density. Use the formula: mass = volume × density.

Convert the mass of methanol to moles using its molar mass. Methanol (CH<sub>3</sub>OH) has a molar mass of approximately 32.04 g/mol. Use the formula: moles = mass / molar mass.

Calculate the mass of the solvent (water) in kilograms, as molality is expressed in moles of solute per kilogram of solvent.

Calculate the molality of the solution using the formula: molality = moles of solute / kilograms of solvent.

Verified Solution

Video duration:

This video solution was recommended by our tutors as helpful for the problem above.

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Concentration Units

Concentration can be expressed in various units, including molarity (moles of solute per liter of solution) and molality (moles of solute per kilogram of solvent). Understanding these units is crucial for accurately calculating the concentration of a solution, as they reflect different aspects of solute and solvent relationships.

Molarity vs. Molality

Molarity is defined as the number of moles of solute divided by the volume of the solution in liters, while molality is defined as the number of moles of solute divided by the mass of the solvent in kilograms. The distinction is important because molarity can change with temperature due to volume expansion, whereas molality remains constant as it depends solely on mass.

Density and Volume Calculations

Density is the mass per unit volume of a substance, and it is essential for converting between mass and volume. In this problem, the densities of methanol and water allow for the calculation of the mass of each component, which is necessary for determining molality, as it requires the mass of the solvent.

Recommended video:

Density Concepts

Related Practice

Describe how to prepare each solution from the dry solute and the solvent. b. 125 g of 0.100 m NaNO₃

Describe how to prepare each solution from the dry solute and the solvent. c. 125 g of 1.0% NaNO₃ solution by mass

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

Household hydrogen peroxide is an aqueous solution containing 3.0% hydrogen peroxide by mass. What is the molarity of this solution? (Assume a density of 1.01 g/mL.)

1802

views

rank