Question

Assertion : One molal aqueous solution of glucose contains $180g$ of glucose in $1kg$ water .
Reason : The solution containing one mole of solute in $1000g$ of solvent is called one molal solution.
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: Molality is a method of expressing concentration of a solution . Molality is defined as the number of moles of the solute dissolved per kilogram of the solvent .

Complete answer:
We have to solve this question using molality .
The formula for molality is
$Molality = \dfrac{n}{{{M_{solvent}}}}$
Where , n = number of moles of solute and ${M_{solvent}} = $ mass of the solvent in kilograms
Now , $n = \dfrac{w}{M}$
where , w = given mass of solute and M = molecular mass of solute
Now , first we will find out the number of moles in $180g$ of glucose
$ \Rightarrow n = \dfrac{{180}}{{180}} = 1$ (since molecular mass of glucose is $180g/mol$ )
So 1 mole of glucose is dissolved in 1 kg of the solvent
Hence , its molality will be ( when number of moles = 1 and mass of solute is 1kg )
$Molality = \dfrac{1}{1} = 1$
Hence , reason is correct that one molal aqueous solution of glucose contains $180g$ of glucose in $1kg$ water .
Also we know that $1kg = 1000g$
Therefore assertion is also correct that the solution containing one mole of solute in $1000g$ of solvent is called one molal solution.
So , we can say that Both assertion and reason are correct and reason is the correct explanation for assertion .
Hence option A is correct .

Note:
Molality is the most preferred way of expressing concentration of a solution because it involves masses of solute and solvent in its calculation which do not change with temperature whereas molarity , normality and others involve volumes of the solutions which change with temperature .