Question

If 4g of NaOH is dissolved in 100mL of aqueous solution, what will be the difference in its molarity and normality?

Answer 1

Hint: Normality is the gram equivalent weight of solute per litres of solvent and molarity is the number of moles of solute per litres of solvent. Here, we can calculate molarity by finding out the number of moles of NaOH and then dividing it by the valency factor will give us the normality too.

Complete answer:
We use both normality and molarity to explain the concentration of a given solution.
The difference between molarity and normality is that molarity is the number of moles whereas normality is the number of equivalent weights of solute per litre of solvent.
In the question, it is given to us that 4g of sodium hydroxide is dissolved to form 100ml of an aqueous solution.
Firstly, let us find out the molar mass of sodium hydroxide.
We know that atomic weight of sodium = 23
Atomic weight of oxygen = 16 and
Atomic weight of hydrogen =1
Therefore, molar mass of sodium hydroxide, which is NaOH = 23+16+1 = 40.
Now, we can calculate the number of moles of NaOH in the solution by dividing the amount of NaOH added by the molar mass of sodium hydroxide.
Therefore, number of moles = $\dfrac{4g}{40g/mol}=0.1mol$
Now we need to convert 100 mL to litres. We know that-
1L = 1000 mL
Therefore, 100mL = 0.1 L

Now we can write that, molarity = $\dfrac{number\text{ }of\text{ }moles\text{ }of\text{ }solute}{litres\text{ }of\text{ }solvent}=\dfrac{0.1}{0.1}M = 1M$.
From the above calculation, it is clear that the molarity of the solution is 1M.
Now, for normality, we can use the formula, Normality = Molarity $\times $ valency factor.
We know that sodium hydroxide is monobasic. Therefore, the valency factor of sodium hydroxide is 1.
Therefore, normality = 1 $\times $ 1 = 1N.
Here, as we can see both normality and molarity are the same, 1. Hence, there is no difference between normality and molarity in the given solution.

Note: There are different terms used to denote the concentration of a solution, they are normality, molarity and molality. We should not be confused between them. Molality is the number of moles of solute per kilogram of the solvent and we have already discussed the other two above.