Solute, solution, solvent and concentration formula:

Solute: A substance that is dissolved. For example, the solute of sugar water is sugar, and the solute of salt water is salt.

Solvent: Something (mostly liquid) used to dissolve a solute. For example, water in sugar water and salt water.

Solution: A mixture of solute and solvent. Such as sugar water and salt water.

The relationship between them: solution mass = solute mass+solvent mass.

Concentration: the ratio of solute mass to solution mass, usually expressed as a percentage.

Basic formula: concentration = solute mass ÷ solution mass × 100%= solute mass ÷ (solute mass+solvent mass) × 100%.

Deformation formula: solute mass = solution mass × concentration, solution mass = solute mass/concentration.

When doing the problem, we should pay attention to distinguish which solute concentration the question asks. For example, sugar and salt are put into water at the same time. If the concentration of sugar is determined, sugar is a solute. If you ask about the concentration of salt, then salt is a solute.

Basic question type: the application of direct examination formula

Problem-solving skills: carefully examine the questions, find out the solute, solvent and solution, and directly apply the formula to answer.

Example 1: Put 50g of salt into 150g of water. What is the concentration of brine?

Thinking: salt is soluble in water, 50g salt is solute, 150g water is solvent, and solution = salt mass+water mass = 50+ 150 = 200g.

Solution: concentration = 50 ÷ (50+150) ×100% = 25%.

A: The concentration of this brine is 25%.

Example 2: How many grams of water should be added to prepare saline water with a salt content of 5% with15g salt?

Idea: directly use the deformation formula of concentration formula to calculate the quality of solution, that is, the quality of brine. Then subtract the mass of salt.

Solution: brine mass = solute mass ÷ concentration = 15÷5%=300 (g)

Water quality = salt water quality-salt quality =300- 15=285 (g)

A: It needs 285 grams of water.

Advanced question type: existing solution, change concentration.

Example 3: Add100g of water to 400g of salt water with a salt content of 5%. What is the salt content of salt water at this time?

Thinking: When 100g water is added, the quality of salt as solute remains unchanged, which can be obtained by 400×5%; The mass of the solution used to be 400g, but now it is (400+ 100g). Then use the concentration formula to solve.

Solution: Salt content = 400× 5% ⊙ (400+100 )×100% = 4%.

A: At this time, the salt content of brine is 4%.

Example 4: There is 20 kilograms of salt water with a salt content of 15%. How many kilograms of salt do you need to add to make the concentration of salt water reach 20%?

Thinking: after adding salt, the mass of solute and solution increases, which is easier to understand with equations. Assuming that X kg of salt is needed, the mass of salt is (20× 15%+x) kg, and the mass of solution is (20+x) kg.

Solution: suppose you need to add salt x kilograms.

(20× 15%+x)÷(20+x)=20%

Solution: x= 1.25

A: You need to add1.25kg of salt.