Question

Number of moles of solute present in 1000 gm of solvent is known as:
A. Normality
B. Molality
C. Molarity
D. Mole fraction

Answer 1

Hint: The number of moles of solute present in 1000 gm or 1 kg of solvent is known as molality or we can say that, Molality = Number of moles of solute / Mass of solvent (in kg)

Complete step by step answer:
In this question we have been asked about what is the no. of moles in a 1000g or 1 kg of solvent called. Our first option is normality. Normality is described as the number of gram or mole equivalents of solute present in one liter of a solution. Equivalent means the number of moles of reactive units in a compound.
Normality = Number of gram equivalents of solution / volume of solution (in L)
$N = \dfrac{{gram equivalents}}{{V(l)}}$
Our 2nd option is molality. Molality (m) can be defined as the ratio of number of moles of solute to the mass of solvent in kg or we can say the number of moles of solute present in a unit kilogram of solvent.
Molality = Number of moles of solute / Mass of solvent (in kg)
$m = \dfrac{n}{{M(kg)}}$
Our 3rd option is molarity. Molarity (M) can be defined as the ratio of number of moles of solute to the volume of solution in a liter or we can say the number of moles of solute present in a unit litre or cubic decimeter of solution.
Molarity = Number of moles of solute / volume of solution (in L)
$M = \dfrac{n}{{V(l)}}$
Our 4th option is mole fraction. Mole fraction can be defined as the ratio of number of moles of the component in the solution to the total number of moles of all components in the solution.
Mole fraction = Number of moles of solute / Number of moles of solute + Number of moles of solvent
$x = \dfrac{n}{{n + N}}$
So, the correct option will be B i.e Molality.

Note:
The term molality is used to study the properties of solutions related to vapor pressure and temperature changes. It is used because its value does not change with changes in temperature.