Q13.55 P - Chemistry: Molecular Nature Of Matter And Change

Question

When a solution is heated, which ways of expressing concentration change in value? Which remain unchanged? Explain.

Step-by-Step Solution

Answer

When a solution is heated, the ways of expressing concentration that change in value are molarity, molality, mole fraction, parts by mass, and parts by volume.

 

Those that remain unchanged are the molality, mole fraction, and parts by mass.

 

Step 1: Concentration of the Solution

The solution can be formed from the dissolution of the solute in the solvent. The solute decides the nature of the solution.

 

Molarity can be defined as the ratio of the mass of the solute to the volume of the solution present in the liter measure.

$\mathbf{Molarity}\mathbf{=}\frac{\mathbf{Number} \mathbf{of}\mathbf{ } \mathbf{Moles}}{\mathbf{Volume}\mathbf{ } \mathbf{of} \mathbf{the}\mathbf{ } \mathbf{Solution}\mathbf{ } \mathbf{(}\mathbf{in} \mathbf{Litre}\mathbf{)}}$

Molality can be defined as the ratio of the number of moles of the solute in the mass of the solvent in kilogram.

$\mathbf{Molality}\mathbf{=}\frac{\mathbf{Number} \mathbf{of}\mathbf{ } \mathbf{Moles}}{\mathbf{Mass}\mathbf{ } \mathbf{of} \mathbf{the} \mathbf{Solvent} \mathbf{(}\mathbf{in} \mathbf{Kg}\mathbf{)}}$

Mole Fraction can be defined as the ratio of the number of moles of the solute to the number of moles of solvent and the number of solutes.

$\mathbf{Mole}\mathbf{ } \mathbf{Fraction}\mathbf{=}\frac{\mathbf{Number} \mathbf{of} \mathbf{Moles} \mathbf{of}\mathbf{ } \mathbf{One}\mathbf{ } \mathbf{Component}}{\mathbf{Number}\mathbf{ } \mathbf{of}\mathbf{ } \mathbf{Moles} \mathbf{of}\mathbf{ } \mathbf{the} \mathbf{Solvent}\mathbf{+}\mathbf{Number} \mathbf{of} \mathbf{Moles} \mathbf{of} \mathbf{Solute}}$

Parts by mass: It can be defined as the percentage of the ratio of the mass of solute to the total mass of the solution.

$\mathbf{Parts}\mathbf{ } \mathbf{by}\mathbf{ } \mathbf{Mass}\mathbf{=}\frac{\mathbf{Mass}\mathbf{ } \mathbf{of}\mathbf{ } \mathbf{Solute}}{\mathbf{Mass} \mathbf{of}\mathbf{ } \mathbf{the} \mathbf{Solution}}\mathbf{\times }\mathbf{100}$

Parts by volume: It can be defined as the percentage of the ratio of the volume of a salute to the total volume of the solution.

 

$\mathbf{Parts} \mathbf{by} \mathbf{Volume}\mathbf{=}\frac{\mathbf{Volume}\mathbf{ } \mathbf{of} \mathbf{Solute}}{\mathbf{Volume} \mathbf{of} \mathbf{the} \mathbf{Solution}}\mathbf{\times }\mathbf{100}$

Step 2: Expression to calculate the Concentration

The change in the temperature will affect volume whereas there is no change in the mass.

 

The mass of solute, solvent, and solution by the volume is conserved and can expand or contract due to the change in the temperature.

 

As the terms expressing concentration include volume in its calculation and therefore, the concentration will be affected.

Whereas the concentration expression that does not include the volume or they only depend upon the mass of the substance will not be affected.

 

Here, molarity and parts by volume depending upon the volume of the solvent, solute or solution in the calculation of the concentration will show the change in the value as the temperature changes.

 

But the terms like molality, mole fraction, and parts by mass only used mass in calculation and do not change due to temperature difference.