Question

What is the molarity of a solution composed of 5.85 g of potassium iodide, KI, dissolved in enough water to make 0.125 L of solution?

Answer 1

Hint: the concentration of any solution is the amount of solute dissolved in the solvent or the solution. Molarity is a term used to denote the concentration of any solution. It can be defined as the number of moles of solute dissolved per liter of solution. Number of moles can be calculated from the given mass divided by molar mass of the solute.

Complete answer:
We have been given to find the molarity of a solution that contains 5.85 g of potassium iodide in 0.125 L of solution. Molarity is the number of moles of solute dissolved per liter of solution so we will find the molarity in 1 L of solution. For this the number of moles has to be calculated in 0.125 L of solution. Then to find the moles present in 1 L of solution, 1 L volume will be divided by 0.125 L to find the factor with which the moles of KI will be multiplied.
So, number of moles of KI = $\dfrac{given\,mass}{molar\,mass}=\dfrac{5.85\,g}{166\,gmo{{l}^{-1}}}$
Number of moles of KI = 0.03524 moles
Now, the factor by which these moles will be increased is =$\dfrac{1\,L}{0.125\,L}$ = 8
Now, in 1 L of solution the number of moles of KI will be 8 times number of moles in 0.125 L of solution.
So, 0.03524 moles$\times $ 8 = 0.282 moles
These moles are dissolved in 1 L of solution so, they will be 0.282 moles/L or 0.282 M.
Hence, the molarity of KI is 0.282 moles/L or 0.282 M.

Note:
The molarity is calculated in per liter that is 1 L of any solution so we have taken out the factor by which the 0.125 L of solution is made up to 1 L of the solution. The molar mass of KI is the atomic mass of potassium added to atomic mass of iodine, which is 39 + 127 = 166 g per mole. The molarity is denoted by M that means molar.