Question

Assertion
One molar aqueous solution is more concentrated than that of molal aqueous solution.
Reason
Molarity is a function of temperature as volume depends on temperature.
A: Both assertion and reason are correct and reason is the correct explanation for assertion.
B: Both assertion and reason are correct but reason is not the correct explanation for assertion.
C: Assertion is correct but reason is incorrect.
D: Both assertion and reason are incorrect.

Answer 1

Hint: Molarity of a substance is defined as the number of moles of the substance that are present in one liter of the solution and molality is defined as the number of moles that are present per kilogram of the solvent. Solvent is the medium which is more in quantity and solute is the substance which is less in quantity and is dissolved in solvent.

Complete step by step answer:
In this question assertion state that one molar aqueous solution is more concentrated than that of $1$ molal aqueous solution And we know that molarity is number of moles per liter of solution where as molality is number of moles of solute per kilogram of solvent. In the assertion we have given aqueous solution which means that solvent is water. Density of water is $1gc{m^{ - 3}}$ and density is defined as mass of substance per unit volume. If density is $1gc{m^{ - 3}}$ then it means that there is $1g$ of that substance in $1c{m^3}$ volume. So, molality in this case can be defined as the number of moles of solute in $1000mL$ or $1L$ of the solvent. So, now molality can be written as:
Molality$ = \dfrac{{{\text{number of moles}}}}{{1L{\text{ of solvent}}}} = \dfrac{{{\text{number of moles}}}}{{{\text{1Kg of solvent}}}}$
And molarity can be written as:
Molarity$ = \dfrac{{{\text{number of moles}}}}{{1L{\text{ of solution}}}}$
If we see both equations carefully we will observe that the numerator is the same and the denominator is different. If we take $1L$ of solution then it will take volume of both solute and solvent. This means the volume of only solvent is less than $1L$ (as there is volume of solute as well in $1L$ solution). Solvent is water; this means the mass of solvent is equal to the volume of solvent (as the density of water is $1gc{m^{ - 3}}$). This means to make the solution one molal more amount of solvent that is water is needed to be added and it will reduce the concentration of solution. This means concentration of one molar solution is more than the concentration of one molal solution (as to make the solution one molal more amount of solvent is added). This means assertion is correct which states that One molar aqueous solution is more concentrated than that of $1$ molal aqueous solution.
Concentration of a substance is inversely proportional to the volume of solution. According to the osmotic pressure equation $\Pi = iCRT$ where $C$ is concentration and $T$ is temperature, we can conclude that concentration is inversely proportional to the temperature i.e.
$C \propto \dfrac{1}{T}$
And concentration is inversely proportional to volume so volume is directly proportional to temperature i.e.
$V \propto T$
This means reason is also correct which states that molarity is a function of temperature as volume depends on temperature.
But it is not the correct explanation of the assertion.

This means option B is correct which states that both assertion and reason are correct but reason is not the correct explanation for assertion.

Note:
Normality is also used to define the concentration of the solution. Normality is defined as the number of gram equivalents that are dissolved in one liter of solution. Number of equivalents are calculated by multiplying the number of moles of substance with the valence or oxidation state of the substance.