Question

Which of the following has no units?
A.Molarity
B.Normality
C.Molality
D.Mole fraction

Answer 1

Molarity, molality, normality, percent solution, parts per million (ppm) are different methods for expressing solution concentration.

Complete step by step answer:
In order to choose the correct option, we need to understand all the four concepts in detail.
When we are dealing with quantitative analysis of chemicals in chemistry, we need to have an idea about the concentration solution. The basis for expressing solution concentration are molarity, molality, normality, percent solution, parts per million (ppm). Let us try to understand each one of them in detail.
Molarity is the simplest way to identify the concentration of a solution. It can be defined as the number of moles of solute present or dissolved in 1 litre ($1L$) of solution. It is represented by $M$.
$1gram$ in $1000mL$ = $1M$
Mathematically, it can be written as-
$M =$ $number$ $of$ $moles$ $of$ $solute$ $(n)$ $\div$ $volume$ $of$ $solution(L)$
Here, the number of moles of solute can be calculated as (given mass of solute/ molecular mass of solute)
For example, molar mass of sodium hydroxide ($NaOH$) = $23(Na)$ $+$ $16(O)$ $+$ $1(H)$ $=$ $40g$ . So, $1M =$ $40g$ of $NaOH$ dissolved in $1000mL$ of water $({H_2}O)$.It has units of $moles/L$.
Molality can be defined as the number of moles of solute present in $1kg$of solvent. It is represented by $m$ . It has units of $moles/kg$
Normality can be defined as the gram equivalent of solute that can be dissolved in $1d{m^3}$ or $1L$ of solution. The unit of normality is $N$. It is the most preferred method for expressing concentrations during titration calculations.
Mathematically, we can calculate it as,
Normality is numerically defined as molecular mass divided by equivalent mass of solute.
So, $N = \dfrac{{{M_{solute}}}}{{{M_{equivalent}}}}$
By using this above equation, we can calculate the strength of a solution. It is expressed in $g/L$ (grams/litre).

Mole fraction defines the number of molecules of one of the components present in a mixture divided by the total number of moles in that given mixture. The molar fraction is represented by $\chi$ . Suppose a solution consists of components A and B, then the mole fraction will be given by
Mole fraction of solute can be numerically defined as number of moles of solute divided by total number of moles of that solution mixture, i.e., moles of solute + moles of solvent
$\chi = {\dfrac{{{n_A}}}{{{n_A} + n}}_B}$
We can clearly see that it is a unitless quantity, as it represents a fraction of molecules.

So, the correct option is D.

Note:
From the above discussion, we got a very clear understanding that both molarity and molality are different measures of the concentration of solutions. However, molality is usually preferred over molarity concentration, while performing chemistry practical. The reason behind this is that the concentrations in molality, i.e., moles of solute dissolved in one kilogram of solvent is independent of temperature and pressure, while on the other hand this is not so in case of molarity. Molarity changes with change in temperature because the liquid may expand or contract with change in temperature, but molality does not change with temperature because the mass of the solvent is independent of temperature.