Question

The molality of 648 g of pure water is:
(A) 36m
(B) 55.5m
(C) 3.6m
(D) 5.55m

Answer 1

Hint: Molality is defined as the number of moles of solute per kg of solvent. In this question water is acting as both solute as well as solvent. So, first find the number of moles of water, then put the value in the formula taking the mass of solvent also as 648 g.

Formula used: Here we will be using the formula of molality:
$m = {n_{solute}}/mas{s_{solvent}}$
Where, m = molality;
${n_{solute}}$ = moles of solute;
${m_{solvent}}$ = mass of solvent (in kg)

Complete step by step answer:
-First we understand what molality is. Molality is defined as a measure of solute concentration in a solution which means that it is the number of moles of solute per kg of solvent.
Mathematically it is written as: m = moles of solute/mass of solvent
$m = {n_{solute}}/mas{s_{solvent}}$ (When the mass of solvent is in kg)
-According to the question:
Mass of solvent = 648 g = 0.648 kg
Molecular mass of ${H_2}O$= 18 g
Moles = given weight/molecular weight = 648/18
 = 36 mole
Molality, m = 36/0.648
= 55.55m
So, the correct option is: (B) 55.5 m

Note: Sometimes we often get confused between molarity and molality since both are measures of chemical solution concentrations. Molarity describes the moles of solute in relation to the volume of the solution while molality describes with the moles of solute in relation to the mass of the solvent.