Question

What will be the molarity of $20\%$ (by mass) $NaOH$ solution if the density of the solution is $1.2gm/cc$ ?
A.$6M$
B.$3M$
C.$0.5M$
D.$5M$

Answer 1

The composition of a solution is important to determine the feasibility of a solution and the composition of a solution can be determined by calculating the concentration of the solution which can be calculated by various methods. Molarity is a method to calculate the concentration of a solution and remember that mass percentage of a solute is taken in 100g solution.

Formula used: $M = \dfrac{{moles\, of\, solute}}{{volume\, of\, solution(L)}}$

Complete step-by-step answer: The composition of a solution is important to determine the feasibility and applications of the solution and the reaction in which they are used. The composition of a solution can be determined by calculating the concentration of the solution. It can be calculated either quantitatively or qualitatively. The qualitative analysis depends on the amount of solute in the solution, if the solute is present in large quantities then the solution is said to be concentrated otherwise the solution is diluted. There are various methods to calculate the quantitative value like mass percentage, mole fraction, molarity and others.
Molarity is a method for quantitative analysis of concentration of a solution. It is defined as the ratio of number of moles of solute in a solution to the volume of the solution.
$M = \dfrac{{moles\, of\, solute}}{{volume\, of\, solution(L)}}$
For the given question,
$20\% NaOH$ means that it is present in $100g$ solution.
$Volume\, Of\, Solution = \dfrac{{mass\, of \,solution}}{{density}}$
Density is given to us as $1.2gm/cc$
$\Rightarrow V = \dfrac{{100}}{{1.2}}$
$\Rightarrow V = 83.33cc$
Mass of solute = $20g$ molecular mass of $NaOH = 40$
$M = \dfrac{{\dfrac{m}{{{m_o}}}}}{{V(L)}}$
$\therefore M = \dfrac{{20 \times 1000}}{{40 \times 83.33}}$
$\Rightarrow M = 6$

Thus, the correct option is (A) $6M$ .

Note: The solubility of a substance is defined as the maximum amount of solute which can be dissolved in a solvent. The solubility of a solution depends on the nature of the solute and the solvent and also on the temperature and pressure. Like dissolves like and therefore, a solute dissolves in a solvent when the intermolecular forces are similar.