"Concentration of Solution" Solution PPT Courseware 5

"Concentration of Solution" Solution PPT Courseware 5

1. Mass fraction of solute

1. Definition: The mass fraction of solute in a solution is the ratio of the mass of the solute to the mass of the solution.

2. Calculation formula:

Mass fraction of solute = mass of solute/mass of solution × 100%

Comparison of solute mass fraction and solubility

The mass fraction of solute 36% means: 100g solution contains 36g solute.

The solubility of a solute at TOC is 36g. This means that at TOC, 100g of water can dissolve up to 36g of the solute.

Solute mass fraction of saturated solution = solubility/100g + solubility × 100%

practice

At a certain temperature, the solubility of substance A is 60g, then the mass fraction of solute in 80g saturated solution at this temperature is ( )

A. 60% B. 80% C. 30% D. 37.5%

Class exercises

1. What is the wrong understanding of the meaning of "10%" salt solution" (AD)

A. 10g of salt dissolved in 100g of water

B. 10g of salt is dissolved in 100g of salt solution

C. A solution obtained by dissolving 10g of table salt in 90g of water

D. Make a solution of salt and water at a mass ratio of 1:10

How do the masses of the solution, solute, and solvent change when water is added to a 20% concentrated solution? How to solve calculations regarding solution dilution?

Basis for calculation:

Mass of solute before dilution = Mass of solute after dilution

1. Solution dilution

Add water to dilute:

The mass of the solute remains unchanged before and after dilution

The total mass of the diluted solution = the mass of the original solution + the mass of water

Formula: m thick × a% (rich) = m thin × b% (thin)

m thick × a% (thick) = (m thick + m water) × b% (thin)

Dilute with dilute solution:

Total mass of solute in the mixed solution = mass of solute in concentrated solution + mass of solute in dilute solution

Calculation formula:

m thick × a% (rich) + m thin × b% (thin) = (m thick + m thin) × c% (thin)

Example 6: It is known that 19.5 g zinc can react completely with 150 g dilute sulfuric acid. beg:

(1) The quality of hydrogen produced;

(2) The mass fraction of solute in dilute sulfuric acid before reaction;

(3) The mass fraction of solute in the solution obtained after the reaction.

Analysis: Find the mass fraction of solute in the resulting solution. This kind of question should be solved using the "holistic method". First find the mass of the solute, and then use the overall method for the mass of the solution. Imagine that all substances react in a container. You add up the mass of the things you put in, and then subtract the generated gas and precipitation (solid) (possibly are unreactive impurities), and what is left is the mass of the solution.

practice

1. 3.25 grams of zinc react completely with 100g of hydrochloric acid. Find:

(1)The quality of hydrogen produced

(2) Mass fraction of hydrochloric acid solute.

(3) Solute mass fraction of the solution obtained after the reaction

2. Concentration of solution

Methods to increase the concentration of a solution, that is, to increase the mass fraction of the solute, include adding solute, evaporating the solvent, and mixing with a concentrated solution of the same solute.

(1) Increase solute

The mass of the solute in the solution = the mass of the solute in the original solution + the increased mass of the solute

Calculation formula: m dilute × a% + m mass = (m dilute + m mass) × b %

(2) Evaporate the solvent

Before and after evaporation of the solvent, the mass of the solute remains unchanged

Calculation formula: m dilute × a% = (m dilute - m water) × b %

(3) Mix with a concentrated solution of the same solute:

Total mass of solute = mass of solute in concentrated solution + mass of solute in dilute solution

m thin × a % + m thick × b % = (m thin + m thick) × c %

Practice: There are 50g of potassium nitrate solution with a mass fraction of 5%. If the solute mass fraction is increased to 10%, the method that can be used is: ( )

A. Evaporate 25g of water

B. Evaporate 23.75g of water

C. Add 2g potassium nitrate crystals

D. Add 50g potassium nitrate solution with a solute mass fraction of 5%

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