Question

Assertion: Molarity of a solution does not depend upon temperature whereas molality depends.
Reason: Molarity and molality both depend only on the number of moles of solute particles.
a.) Both Assertion and Reason are correct and Reason is the number 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: Before attempting this question one should have prior knowledge about the relation of molality and molarity with temperature and also remember that molarity and molality are mostly related to numbers of moles of solute, use this information to approach towards the solution.

Complete step by step solution:
Molarity of a solution depends upon temperature whereas molality does not depend upon the temperature
Reason: Molarity and molality both depend only on the numbers of moles of solute particles.
Molarity and molality may sound similar but they both are different by means of definition.
Molarity is the number of moles of solute which are dissolved per liter of solution, i.e.
$C = \dfrac{{{n_{solute}}}}{{{V_{solution}}}}$
The definition clearly stated that molarity depends on volume, and as we all know if the temperature increases then volume also increases. Similarly, if we decrease the temperature the volume decreases. This happens due to the molecules of the gas will have more kinetic energy if we increase the temperature and if the container can expand then the volume increases.
Therefore, we can say molarity depends on temperature.

Now, talking about molality, molality is the number of moles of solute per kilogram of the solvent, [not solution]. i.e.
$b = \dfrac{{{n_{solute}}}}{{{m_{solvent}}}}$

When we increase or decrease temperature neither the number of moles of solute nor mass of solvent will be affected by change of temperature.
Hence, we can conclude that molality doesn’t depend upon temperature.
The reason given in the question is also incorrect because molarity and molality doesn’t depend upon no. of moles of solute particles alone.
So, the correct answer is “Option D”.

Note: The term molarity and molality can be explained as molality defines the amount of substance present in the given mass of solvent or we can say that it tells about the number of moles of solute that are present in per kilogram of given solvent whereas molarity explains the amount of substance that is present in the given solution’s volume.