Question

Calculate the molarity of $KI$ if the density of $20\% $(mass/mass) aqueous solution of $KI$ is $1.202gm/L$.

Answer 1

Hint: Molarity of a compound in its aqueous solution is number of moles per litre of solution. First find the mass of $KI$ in one litre of solution by using given information. Then, divide the mass of $KI$ with its molar mass and give us the number of moles of $KI$ in one litre of its aqueous .

Complete step by step answer:
Density of aqueous solution of $KI$ is $1.202gm/L$.
Let we have one litre of solution, then the mass of one litre solution is $1.202gm$.
In the given solution $KI$ is $20\% $(mass/mass), then the mass of $KI$ in solution is $\dfrac{{20}}{{100}} \times 1202 = 240.4gm$.
Molar mass of $KI$ is $39 + 127 = 166gm/mol$.
We know that molarity of a solution is the number of moles per litre of solution or molarity is also equal mass of compound in one litre of solution divided by molar mass.
Then, ${\text{molarity of KI = }}\dfrac{{{\text{mass of KI}}}}{{{\text{molar mass of KI}}}} = \dfrac{{240.4}}{{166}}$
$Molarity = \dfrac{{240.4}}{{166}} = 1.445mol/L$
Hence molarity of $KI$ is $1.445mol/L$.
Hence the correct answer is option D (violet).

Note:We know that some elements have their isotopes (same element with different molar mass) due to which molarity of compound may have some error. For example, there are two main isotopes of potassium, $^{39}K(93.3\% )$ and $^{41}K(6.7\% )$ occurs naturally and have different molar mass. Due to different molar mass, molarity may change with change in percentage of isotopes. Molarity also depends on volume and volume depends on the temperature of solution, then molarity also depends on the temperature of solution. If we change the temperature of solution, molarity also changes.