Chemistry, 04.02.2020 20:54 Jaimewillis42

What are the differences in measuring the molar mass using freezing point depression between studying ionic solids and molecular solids?

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Answer from: Isa035

The molar mass measured using freezing point depression is more for ionic solids as compared to that of molecular solids.

Further Explanation:

Colligative properties are the properties that depend on the number of particles and not on the mass and identity of the solute. The four colligative properties are as follows:

1. Relative lowering of vapor pressure

2. Elevation in boiling point

3. Depression in freezing point

4. Osmotic pressure

The expression for the freezing point depression is as follows:

$\Delta {{\text{T}}_{\text{f}}} = {\text{i}}{{\text{k}}_{\text{f}}}{\text{m}}$

…… (1)

Here,

$\Delta\text{T}_{\text{f}}$ is the depression in freezing point.

$\text{K}_{\text{f}}$ is the molal freezing point depression constant.

i is the van’t Hoff factor.

m is the molality of the solution.

Rearrange equation (1) to calculate m.

${\text{m}} = \dfrac{{\Delta {{\text{T}}_{\text{f}}}}}{{{\text{i}}{{\text{k}}_{\text{f}}}}}$ …… (2)

Molality is defined as the amount of solute dissolved in 1 kg of the solvent. It is denoted by m. The expression to calculate the molality of the solution is as follows:

${\text{m}} = \dfrac{{{\text{Amount}}\left( {{\text{mol}}} \right){\text{of solute}}}}{{{\text{Mass}}\left( {{\text{kg}}} \right){\text{of solvent}}}}$ …… (3)

Rearrange equation (3) to calculate the amount of solute.

${\text{Amount}}\left( {{\text{mol}}} \right){\text{of solute}} = \left( {\text{m}} \right)\left( {{\text{Mass}}\left( {{\text{kg}}} \right){\text{of solvent}}} \right)$

…… (4)

The molar mass can be calculated as follows:

${\text{Molar mass of solute}} = \left( {{\text{Amount of solute}}} \right)\left( {{\text{Mass of solute}}} \right)$ …… (5)

Incorporate equation (4) into equation (5).

${\text{Molar mass of solute}} = \left( {\text{m}} \right)\left( {{\text{Mass of solvent}}} \right)\left( {{\text{Mass of solute}}} \right)$ …… (6)

Equation (6) indicates the direct relationship between the molar mass and the molality of the solution. Higher the molality, more will be the molar mass and vice-versa.

Van’t Hoff factor shows the effect of solute particle son the colligative properties. It is represented by i. For molecular compounds, the value of i is one and for ionic solids, it is equal to the number of ions produced by the compound.

Molecular solids cannot dissociate into its constituent particles on ions when dissolved in the solvent. So these exist as a single entity and therefore the value of van’t Hoff factor is 1. But ionic solids dissociate into its ions on dissolution. The number of ions produced will always be two or more than two and so as the value of van’t Hoff factor.

Since the value of van’t Hoff factor for ionic solids is always greater than for molecular solids, its molar mass will be less than that of molecular solids.

Learn more:

What is the concentration of alcohol in terms of molarity? What is the molarity of the stock solution?

Answer details:

Grade: Senior School

Chapter: Solutions

Subject: Chemistry

Keywords: colligative properties, depression in freezing point, relative lowering of vapor pressure, osmotic pressure, elevation in boiling point, mass, identity, number, m, , Kf, ionic solids, molecular solids.

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