Chemistry middle school to high school arithmetic formula, chemistry junior high school calculation

In this problem, you need to understand the chemical relationship in the problem, and convert the language of chemical relations into mathematical language, which is usually to solve a system of binary equations.

There is a certain routine for general calculations, and it is necessary to find the rules of doing the problem and summarize this routine and solve it step by step.

In fact, the chemistry calculation questions in junior high school are very simple, focusing on the following aspects.

1.Read the meaning of the question and clarify the test point of the question.

2.Be able to write the information points in the question.

3.Able to write equations correctly.

4.Able to find known and unknown quantities.

5.It can list proportional formulas according to the question information combined with known quantities and unknown quantities.

6.The final result can be calculated.

7.Be able to appreciate the details examined in the question.

Middle School Chemistry.

Commonly used calculation formulas.

One. Commonly used calculation formulas.

1) Relative atomic Mass =

The mass of an atom of an element.

1 12 of the mass of a carbon atom

2) Let the chemical formula of a compound be AMBN

Its relative molecular mass Relative atomic mass of a m Relative atomic mass of b Mass ratio of element A to element b Relative atomic mass of a m: Relative atomic mass of b Mass fraction of element A = Relative atomic mass of a m Relative molecular mass of ambn.

3) The mass fraction (purity) of a substance contained in the mixture = the mass of the pure substance The total mass of the mixture.

4) Gas density (g l) = gas mass (g) gas volume (l) (5) purity = mass of pure substance Total mass of the mixture.

The mass of the pure substance (the mass of the pure substance + the mass of the impurities) 100%=

1- Mass fraction of impurities.

6) Mass fraction of solute = mass of solute Mass of solution.

Solute mass (solute mass + solvent mass).

7) Dilution and concentration of the solution

a% concentrated = m diluted.

b% dilute = (m concentration + increased solvent mass).

b% rare. 8) Mix two solutions with different mass fractions relative to solutes: m concentration: a%, concentration + m dilution.

b% dilute = (m concentrated + m diluted) c%.

9) The quality of the solute in the solution.

The mass fraction of the solute in the solution.

The volume of the solution The density of the solution.

Generate precipitation, calculate the gram of magnesium chloride, because it is general, so the original solution is 19 grams, then add 200 grams of water is the total mass of magnesium chloride solution is 219 grams, calculate the mass of sodium hydroxide is 8 grams, because the mass fraction is 20%, so the mass of sodium hydroxide solution is 40 grams.

Therefore, the mass fraction of solute in the solution formed after the reaction is [219+].

3. Calculation of chemical formula: (1) The sum of the relative molecular mass (the number of atoms corresponding to the relative atomic mass).

2) The ratio of the mass ratio of the element (the number of atoms corresponding to the relative atomic mass). (3) The mass fraction of the element is calculated by taking Fe2O3 as an example.

4. The mass fraction of an element in the compound = the mass of the compound The mass fraction of the element.

5. The mass fraction of an element in the mixture = the mass of the mixture The mass fraction (purity) of the substance The mass fraction of the element. Purity The quality of the pure substance The quality of the mixture is 100

The formula for calculating the average molar mass or the average quantity.

1) The total mass m (mixed) of the known mixture and the amount of total substance n (mixed):

Note: This method of finding the average molar mass of a mixture is not only applicable to gases, but also to solids or liquids.

2) The density of the gas mixture under known standard conditions.

Mixed) :(mixed).

Note: This method is only suitable for gas mixtures under standard conditions (0).

3) It is known that at the same temperature and pressure, the ratio of the density of the mixed gas to the density of another gas a d (usually called the relative density): then.

Calculations about the solution:

1.Common types of calculations are as follows:

1) Knowing the mass of the solute, the mass of the solvent and the mass fraction of the solute, find the other quantity;

2) Calculation of dilution and concentration of solution.

3) the calculation of the mass fraction of solute after mixing of solutions with different solute mass fractions;

4) Prepare a solution with a certain mass fraction of solute, and find the amount of solute and solvent required.

2.Method guidance: Questions about the calculation of the mass fraction of solutes.

1) It is necessary to closely follow the definition and mathematical expression of the mass fraction of the solute: the mass of the solute The mass of the solution * 100%.

2) To distinguish what the solute and solvent are in the solution, analyze and find out the mathematical expression according to the known quantity in the question.

The three quantities: the mass of the solute, the mass of the solvent or solution, and the mass fraction of the solute, are then substituted into the formula for calculation.

3) When calculating, we should pay attention to the two universes of "mass", the data we use must be the mass of each substance, and the mass is also found, if the quantity known or sought is volume, it must be converted appropriately according to the formula: density mass volume.

4) Solution dilution and concentration problems: to grasp the key that the quality of the solute before and after dilution and concentration remains unchanged, the quality of the solute before and after dilution and concentration is expressed by known and unknown quantities, and the equation containing unknown quantities is listed, and the solution can be solved.

5) When two solutions with different mass fractions of solutes are mixed, the sum of the masses of the first two solutions is equal to the mass of the solution obtained after mixing; The sum of the masses of the solutes in the two solutions before mixing is equal to the mass of the solutes in the mixed solution. will mix.

The mass of the solution and solute is substituted into a formula for calculation.

6) Calculation of the mass fraction of solutes in the solution of the product: The focus is on the calculation of the solution mass and solute mass of the product. The reaction of some substances with water is easily overlooked, resulting in an incorrect calculation of solute mass, and some reactions are due to the formation of precipitation.

Gases may contain impurities, making the mass of the solution difficult to calculate. If the product is only water and the reactant does not contain impurities, the mass of the product solution is equal to the sum of the mass of the reactant solution; If the product has gas or precipitate, the mass of the product solution is equal to the mass of the reactant solution minus the mass of the gas or precipitate.

There are two formulas, one is to find the saturated solution and the other is to find the unsaturated solution. S 100 + S = m substance M liquid, S 100 = M substance M agent.

Relative mass of A: Relative mass of B = Actual mass of A: Actual mass of B.

3. Calculation of chemical formula: (1) The sum of the relative molecular mass (the number of atoms corresponding to the relative atomic mass).

2) The ratio of the mass ratio of the element (the number of atoms corresponding to the relative atomic mass). (3) The mass fraction of the element is calculated by taking Fe2O3 as an example.

4. The mass fraction of an element in the compound = the mass of the compound The mass fraction of the element.

5. The mass fraction of an element in the mixture = the mass of the mixture The mass fraction (purity) of the substance The mass fraction of the element. Purity The quality of the pure substance The quality of the mixture is 100

The calculation of the junior high school chemistry test is generally based on the following question types: 1 is to calculate the relative molecular mass according to the chemical formula, 2 is to calculate the ratio of the number of atoms of an element, 3 is to calculate the mass ratio of an element, and 4 is to calculate the mass fraction of one element. What kind of question are you asking, you should write the question, otherwise there are too many questions and others know how to help you?

What type of ah, I forgot the books in junior high school, I majored in chemistry in college, you can tell me more about it, and see if I can help you.