Question

Assertion: Molarity of pure water is $55.55M$ at $298K$ .
Reason: Molarity is temperature dependent.
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. Assertion is incorrect but reason is correct.

Answer 1

Hint : Molarity is in the term of volume, in simple words it is dependent on volume of the solution. As volume is temperature dependent of temperature hence, molarity is also temperature dependent. Molarity is also known as molar concentration of solution.
$M = \dfrac{n}{v}$
Where,
$M$ = Molarity of solution
$n$ = Number of moles of solute
$v$ = Volume of solution (in $litres$ ).

Complete Step By Step Answer:
Molarity is defined as the ratio number of moles of solute and volume of solution in $liters$ . In other words, molarity is the amount of a substance in a certain volume of solution. It is the number of moles of solute per $liters$ of a solution.
Now, according to the question:
Mass of $1liter$ of solution is $1000g$ .
Molar mass of water is $2 \times 1 + 16$ $\Rightarrow 18$
Moles of water: $\dfrac{{1000}}{{18}} \Rightarrow 55.55$
So, molarity= $\dfrac{n}{v}$
$\Rightarrow \dfrac{{55.55}}{1}M$
$Molarity = 55.55M$ , at $298K$ .
So, as we can see, molarity is termed as volume so it is temperature dependent.
Here, assertion and reason both are correct but reason is not the correct explanation for assertion.
So, option B is correct.

Additional Information:
There is a relative quantity to molarity that is termed as molality, molality is defined as the ratio of number of moles of solute and mass of solvent in kilogram. In other words it is simply the number of moles of solute present in one kilogram of solvent. This is differ from molarity and it is represented by $'m'$ .

Note :
In thermodynamics the use of molar concentration is often not preferred because mostly the volume of solutions slightly depends on temperature due to thermal expansion. This problem can be resolved by introducing temperature correction factors, or by using a temperature-independent measure of concentration such as molality. The reciprocal quantity represents the dilution (volume) which can appear in Ostwald's law of dilution.