Question

When 400 g of a 20% solution was cooled 50 g of the solute precipitated. What is the percent concentration of the remaining solution.

Answer 1

When a solute is added in a solvent and it remains insoluble then the solute settles down at the base which is known as precipitate and the process is known as precipitation. Solution is a mixture of solute and a solvent. Solution is a heterogeneous mixture. Solute is a substance or a compound which is added to a solvent or a liquid. Solvent is a substance or a compound in which a solute or a compound is mixed. We shall calculate the mass of solute present in the solution originally and use it to find the mass of solute in the final solution.

Complete step by step solution:
The given weight of the solution is equal to 400 g.
The weight of the solute added in the solution is equal to the 20 percent of total weight of the solution. The weight of the solute added in the solution = $\dfrac{{20}}{{100}} \times 400$ = 80 g
The given weight of solute precipitated during the process is equal to 50 g. Hence,
Weight of the solute which is still dissolved in solvent = Weight of solute added initially - Weight of the solute precipitated out.
Therefore, weight of solute which is still dissolved the solvent is = $80 - 50 = 30{\text{ g}}$
The weight of the remaining solution = Total weight of the solution - Weight of the solute precipitated
Therefore, the weight of the solute left = $400 - 50 = 350{\text{ g}}$
Hence the percentage concentration of the remaining solution is the remaining dissolved solute that is 30 grams divided by the total mass of the remaining solution that is 350 grams multiplied by 100 to get the percentage.
Therefore,
Percentage concentration of remaining solution = $\dfrac{{30}}{{350}} \times 100$
Percentage concentration of remaining solution = $8.57$ %
Therefore, the final answer is $8.57$ percentage.

Note:
When a compound or substance or a solute is added to a solvent and the solute dissolves or gets mixed with the solvent then the solute is called a soluble solute. When a compound or a substance or a solute is added to a solvent and the solute does not dissolve in the solution or does not get mixed with the solvent then the solute is called an insoluble solute.