Many students did not perform satisfactorily in the exam. The main reason for this problem is that there are not only things that the school has learned, but also many problems that the school has not talked about. Children have no idea when they get this kind of question, and they probably haven't written the whole question step by step. This time, let's discuss the concentration of key questions in Xiaoshengchu.
~ the basic quantity in the concentration problem ~
solute
Liquid-soluble substances (usually? Salt, sugar, alcohol? )
solvent
A liquid that dissolves substances (usually? Water? )
solution
Mixed solution of solute and solvent
concentrate
The percentage or percentage of solute in a solution.
(Percentage of salt in brine)
~ basic quantitative relationship ~
Solution = solute+solvent
Solution? Concentration = solute
Solute? Concentration = solution
Solvent = solution? (1- concentration)
Concentration = solute? Solution? 100%= solute? (Solute+Solvent)? 100%
~ the basic method to solve the concentration problem ~
① Concentration and dilution: Grasp the invariants.
(2) the proportion of the solution: the solution of the equation.
Intensive analysis of test sites
Roast point 1
Simple matching problem:
Example 1
The concentration of alcohol solution obtained by mixing 500g of 70% alcohol solution with 300g of 50% alcohol solution is ().
Fine analysis
Understand the concept of concentration and directly apply the concentration formula concentration = solute? Solution? 100%。
answer
Solution:
* * * Alcohol: 500? 70%+50%? 300=500 grams
Concentration: 500? (500+300)=62.5%
Inductive summary
Apply the concentration formula directly, and don't confuse the formula.
Example 2
How many kilograms can 30% solution and 10% solution be mixed into 26% solution?
Fine analysis
Understand the concept of concentration and directly apply the concentration formula.
answer
Solution:
4? 30% + 10%x=(4+x) 26
x= 1
A: We should take 10% solution 1 kg.
Inductive summary
Apply the concentration formula directly, and establish the equation by grasping the invariants or looking for equivalence relations.
Kaoxian 2
Concentration problem (constant solvent, increased solute):
example
600 grams of sugar water, sugar content 7%. How many grams of sugar do you need to add to increase the sugar content to 10%?
Fine analysis
When the solvent remains unchanged, the solute increases and the water (solvent) remains unchanged.
answer
Solution: the quality of water in raw sugar water;
600? (1-7%)=558 (g)
Now the quality of sugar water:
558? (1- 10%)=620 (g)
Mass of added sugar: 620-600=20 (g)
You need to add 20 grams of sugar
Inductive summary
Master the invariant solution
Kaoxian 3
Dilution problem (solute unchanged, solution increased):
example
The fruit warehouse shipped 400 kilograms of a fruit with a water content of 90%. After another week, it was found that the water content was reduced to 80%. What is the total weight of this batch of fruit now?
Fine analysis
The quality of pulp has not changed.
answer
Solution: treat fruit as? Solution? , in which water is considered to be? Solute? ,
Fruit as? Solvent? , water content is? Concentration? .
Before the change? Solvent? The weight of is:
400? (1-90%)=40 (kg),
After the change? Solution? The weight of is:
40? (1-80%)=200 (kg)
A: Now the total weight of this batch of fruit is 200 kilograms.
Inductive summary
Grasp the invariant and answer the invariant body.
Kaoxian 4
Proportioning problem of different concentrations (equation);
example
600g of15% brine was prepared by mixing 20% brine with 5% brine. How many grams do 20% saline and 5% saline need?
Fine analysis
Find the equivalence relation, and the solute has not changed before and after the ratio.
answer
Solution: Suppose that 20% physiological saline needs x grams,
Then 5% salt water is (600-x) grams.
20%x+(600-x)？ 5%=600? 15%
x=400
600-400=200 (gram)
Inductive summary
Find the equivalence relation of sequence equation
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