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. e. mole percent

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Calculate the mass of methanol using its volume and density: \( \text{mass of methanol} = \text{volume} \times \text{density} = 20.2 \, \text{mL} \times 0.782 \, \text{g/mL} \).

Convert the mass of methanol to moles using its molar mass: \( \text{moles of methanol} = \frac{\text{mass of methanol}}{\text{molar mass of CH}_3\text{OH}} \). The molar mass of methanol (CH₃OH) is approximately 32.04 g/mol.

Calculate the mass of water using its volume and density: \( \text{mass of water} = \text{volume} \times \text{density} = 100.0 \, \text{mL} \times 1.00 \, \text{g/mL} \).

Convert the mass of water to moles using its molar mass: \( \text{moles of water} = \frac{\text{mass of water}}{\text{molar mass of H}_2\text{O}} \). The molar mass of water (H₂O) is approximately 18.02 g/mol.

Calculate the mole percent of methanol in the solution: \( \text{mole percent} = \left( \frac{\text{moles of methanol}}{\text{moles of methanol} + \text{moles of water}} \right) \times 100\% \).

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주요 개념

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Molarity and Concentration Units

Molarity is a common way to express concentration, defined as the number of moles of solute per liter of solution. In addition to molarity, concentrations can also be expressed in terms of mole percent, which indicates the ratio of moles of a specific component to the total moles of all components in the solution, multiplied by 100. Understanding these units is essential for accurately calculating and comparing concentrations.

Density and Mass Calculation

Density is the mass of a substance per unit volume, typically expressed in grams per milliliter (g/mL). To find the mass of a liquid from its volume and density, the formula mass = density × volume is used. This concept is crucial for converting the volume of methanol and water into mass, which is necessary for calculating the number of moles and ultimately the concentration.

Mole Concept

The mole is a fundamental unit in chemistry that quantifies the amount of substance. One mole contains approximately 6.022 x 10²³ entities (Avogadro's number), allowing chemists to relate mass to the number of particles. In this context, calculating the number of moles of methanol and water is essential for determining their contributions to the mole percent of the solution.