## High one chemical difference method

The difference between any two quantities in a chemical reaction is directly proportional to any one of them; The sum of any two quantities is in direct proportion to any one of them. The method of solving the problem by using the above relationship is the difference method or the sum method

### How to calculate by difference method? Tell me about the difference method, with a few examples,

The difference method lists the problem-solving method of proportional formula according to the difference between reactants and products in chemical reaction and the essence and relationship of this difference. Generally speaking, all differences such as mass difference, gas volume difference, density difference and pressure difference before and after chemical reaction can be solved by the difference method. The key to problem-solving is to make clear observation

### Various methods of chemical calculation problems in senior one, such as difference method, etc. be as detailed as possible Difference method Extreme value method Conservation method Cross method

Difference method: 1. Review the meaning of the question and analyze the reasons for the difference. 2. Write the difference on the right side of the chemical reaction equation and take it as the relational quantity. 3. Write the proportional formula and calculate the unknown. Extreme value method: 1. Assume the reversible reaction as a complete reaction to the left or right. 2. Assume the mixture as a pure substance. 3. Parallel

### Common calculation methods in senior one chemistry, such as cross method, difference method, etc,

1、 Difference method

In the saturated solution of a certain amount of solvent, the solubility of solute changes due to the change of temperature (increase or decrease), resulting in the difference in the quality of solute (or saturated solution); Each substance has a fixed chemical composition, and there is a difference between the physical quantities of any two substances; Similarly, for a chemical reaction in a closed system, although the mass is conserved before and after the reaction, the amount of substance, the mass of substances in various states of solid, liquid and gas, and the volume of gas will change to form a difference. The difference method is based on these difference values, A chemical calculation method that lists the proportional formula to solve. The mathematical knowledge used in this method is the law of equal proportion and its derivative formula: or. The difference method is a main means to simplify chemical calculation, which is widely used in middle school. The common types are: poor solubility, poor composition, poor quality, poor volume, poor quantity of substances, etc. pay attention to the same state of substances when using, The unit of physical quantity of different substances shall be consistent

1. Heat 21.0g of the mixture of sodium carbonate and sodium bicarbonate until the mass does not change, and weigh the solid mass as 14.8g. Calculate the mass fraction of sodium carbonate in the mixture

2. In the laboratory, KNO3 is purified by cooling crystallization method. Firstly, KNO3 is prepared into a saturated solution at 100 ℃, and then cooled to 30 ℃ to precipitate KNO3. Now to prepare 500g pure KNO3, ask how many grams of KNO3 should be dissolved in how many grams of water at 100 ℃ (the solubility of KNO3 is 246g at 100 ℃ and 46g at 30 ℃)

3. If the relative molecular weight of the oxide of a metal element R is m and the relative molecular weight of the chloride of the same valence state is n, what is the valence of the metal element R?

4. Put magnesium, aluminum and iron into dilute sulfuric acid with equal mass and sufficient amount respectively, and the mass of each solution obtained after the reaction is equal, then the mass size relationship of magnesium, aluminum and iron is ()

（A）Al＞Mg＞Fe （B）Fe＞Mg＞Al （C）Mg＞Al＞Fe （D）Mg=Fe=Al

2、 Cross method

Any proposition that can be solved by a system of binary primary equations, that is, the average value of two components, can be solved by the cross method. This method transforms the multiplication and division operation into addition and subtraction operation, which brings great convenience to the calculation

The expression of the cross method is deduced as follows: let a and B represent the two components of the cross, represent the average amount synthesized by the two components, Xa and XB represent the percentage of a and B in the average amount respectively, and XA + XB = 1, then:

A • XA + B • XB = (XA + XB) Simplified:

If the is placed in the center of the cross, subtract a and B from their cross, and compare the absolute value of the difference between them to obtain the above formula

The cross method is widely used, but it is not omnipotent. Its scope of application is shown in Table 4-2:

Containing chemistry

Semantic quantity

Type A, B

xA、xB

1 solute in solution

Mass fraction mass fraction mass fraction of solute in mixed solution mass fraction

An element in a substance

Mass fraction of a mixture

Element mass fraction

3 isotope relative

Atomic mass element relative

Atomic mass isotope

Percent composition

4 relative of a substance

Molecular weight average relative molecular weight fraction of mixture

Or volume fraction

5 molecules of a substance

Average composition of the mixture

Mass fraction of molecular composition

6 it is used for some comprehensive calculations, such as determining the composition of some salts and organic substances by cross method

The key to correctly using the cross method lies in: (1) correctly selecting two components and average; (2) Clarify the ratio of income to who; (3) What is the unit in which two substances are compared? In particular, it should be noted that this method is not simple when knowing the average value of mass to calculate the ratio of volume or quantity of substances

1. There are 50g of 5% CuSO4 solution, and the methods to double its concentration are as follows: (1) evaporate g of water from the original solution; (2) 12.5% CuSO4 solution G can be added to the original solution; (3) Add alum g to the original solution; (4) CuSO4 white powder G can be added to the original solution

2. There is a mixed fertilizer of NH4NO3 and CO (NH2) 2. The measured mass fraction of nitrogen is 40%, so the ratio of NH4NO3 to CO (NH2) 2 in the mixture is ()

（A）4∶3 （B）1∶1 （C）3∶4 （D）2∶3

3、 Average method

For quantitative or semi quantitative exercises with average meaning, using the technical method of average principle can save complex calculation, make rapid judgment and skillfully get the answer, which is of great benefit to improve the problem-solving ability. The average method is actually the further application of the principle contained in cross. When solving problems, it is often used in combination with cross to achieve the purpose of quick solution. The principle is as follows:

If a > b, and it is consistent, there must be a > > b, where is the corresponding average value of a and B or formula. Xa • XB is the number of shares of a and B respectively

The common types are: average method of element mass fraction, relative atomic mass, molar electron mass, number of double bonds, chemical composition, etc. sometimes the average method can also be used to discuss the scope

1. If the mass fraction of nitrogen in an ammonium nitrate sample is 25%, a group of impurities mixed in the sample must not be ()

(A) CO (NH2) 2 and NH4HCO3 (b) NH4Cl and NH4HCO3

(C) NH4Cl and (NH4) 2SO4 (d) (NH4) 2SO4 and NH4HCO3

2. After 95mg magnesium chloride powder containing a chloride impurity is dissolved in water, it reacts with a sufficient amount of silver nitrate solution to form 300mg silver chloride precipitation, then the impurity in the magnesium chloride may be ()

(A) Sodium chloride (b) aluminum chloride (c) potassium chloride (d) calcium chloride

3. A CaCO3 sample containing impurities may only contain two of the four impurities in the following brackets. Take 10g of the sample and react with sufficient hydrochloric acid to produce CO2 gas under 2.24l standard condition. Then the sample must contain impurities and may contain impurities. (impurities: KHCO3, MgCO3, K2CO3, SiO2)

4. (1) ammonium bicarbonate decomposes completely at 170 ℃, and the average relative molecular weight of the mixed gas is

(2) If the mass fraction of H2 and O2 in a detonation gas is 75% and 25% respectively, the relative density of the detonation gas to hydrogen is

(3) After the dry container with a volume of 1 L is filled with HCl gas, the relative density of gas to oxygen in the container is measured to be 1.082. Use this gas for fountain experiment. When the fountain stops, the volume of liquid entering the container is

Attachment: calculation method of average molar mass ():

① M total - the mass of the mixture, N Total - the amount of the total substance of the mixture

② = M1 • N1% + M2 • N2% +... M1, M2... Molar mass of each component, N1%, N2%... Quantity fraction of each component. (Note: if it is molar mass of element, M1, M2... Are molar mass of each isotope, N1%, N2%... Are atomic fraction (abundance) of each isotope.)

③ In case of molar mass of gas mixture, there is = M1 • V1% + M2 • V2% +... (Note: V1%, V2%... Gas volume fraction.)

④ If it is the molar mass of the gas mixture, there is = D • MA (Note: Ma is the molar mass of the reference gas, and D is the relative density)

4、 Conservation method

There are a series of conservation phenomena in chemical reactions, such as mass conservation (including atomic conservation and element conservation), charge conservation, electron gain and loss conservation, energy conservation, etc. the method of solving problems by using these conservation relations is called conservation method. Charge conservation means that for any electrically neutral system, such as compound, mixture, solution, colloid, etc., the algebraic sum of charges is zero, That is, the total number of positive charges and the total number of negative charges are equal. The conservation of electron gain and loss means that in the oxidation-reduction reaction, the number of electrons obtained by the oxidant must be equal to the number of electrons lost by the reductant, whether it is the spontaneous oxidation-reduction reaction or the primary battery or electrolytic cell to be learned in the future

a. Conservation of mass

1. There is 0.4g iron oxide, which is reduced by sufficient CO at high temperature, and all the generated CO2 is introduced into sufficient clarified lime water to obtain 0.75g solid sediment. The chemical formula of this iron oxide is ()

A. FeO B. Fe2O3 C. Fe3O4 D. Fe4O5

2. Add the mixture of several iron oxides into 100ml, 7mol • L-1 hydrochloric acid. The oxide is just completely dissolved. When 0.56l (standard condition) chlorine is introduced into the resulting solution, the Fe2 + in the solution is just completely converted to Fe3 +, then the mass fraction of iron in the mixture is ()

A. 72.4% B. 71.4% C. 79.0% D. 63.6%

b. Charge conservation method

3. Put 8g Fe2O3 into 150ml dilute sulfuric acid with a certain concentration, and then put 7g iron powder to collect 1.68l H2 (standard condition). At the same time, there is no residue of Fe and Fe2O3. In order to neutralize excess sulfuric acid and completely precipitate iron in the solution, 150ml of 4mol / L NaOH solution is consumed. Then the amount and concentration of raw sulfuric acid is ()

A. 1.5mol/L B. 0.5mol/L C. 2mol/L D. 1.2mol/L

4. The magnesium strip burns in the air to produce magnesium oxide and magnesium nitride. Dissolve all the products after combustion in 50ml of 1.8 mol • L-1 hydrochloric acid solution, neutralize the excess acid with 20ml of 0.9 mol • L-1 sodium hydroxide solution, then add excess alkali to the solution to release all ammonia, and absorb it with sufficient hydrochloric acid. The ammonia is determined to be 0.006 mol to calculate the mass of the magnesium strip

c. Gain and loss electron conservation method

5. In a dilute nitric acid solution, after adding 5.6g iron powder for full reaction, all the iron powder is dissolved to form No. the mass of the solution increases by 3.2g. The ratio of Fe2 + to Fe3 + in the obtained solution is ()

A. 4∶1 B. 2∶1 C. 1∶1 D. 3∶2

6. (1) 0.5mol copper strip reacts with sufficient concentrated HNO3. After the collected gas is dried (regardless of loss), its density is 2.5G • L-1 and its volume is L

(2) 0.5mol copper sheet reacts with a certain amount of concentrated HNO3. The volume of the collected gas after drying (regardless of loss) under standard conditions is 17.92l, so the amount of nitric acid participating in the reaction is; If these gases are completely absorbed by water, the oxygen volume under standard conditions shall be supplemented to L. (2no2 N2O4 reaction is not considered)

7. It is known that 2 Fe2 + + Br2 = 2 Fe3 + + 2Br -. If 2.24l chlorine under standard condition is slowly introduced into 100mlfebr2 solution, one third of Br - ions are oxidized to br ¬ 2 single substance. Try to calculate the quantity and concentration of the substance in the original febr2 solution

5、 Extreme value method

"Extreme value method", i.e. "extreme hypothesis method", is a common method to solve chemical problems by mathematical methods. It is generally used to solve the calculation of relevant mixtures. It can be assumed that the original mixture is a pure substance, calculate, determine the maximum value and minimum value, and then analyze, discuss and draw a conclusion

1. Under normal temperature, pass a mol H2S and B mol SO2 into a 20L vacuum vessel (A and B are positive integers, and a ≤ 5, B ≤ 5). After the reaction is complete, the maximum density that may be reached in the vessel is about ()

（A）25.5 g•L-1 （B）14.4 g•L-1 （C）8 g•L-1 （D）5.1 g•L-1

2. Under standard conditions, put the gas collecting cylinder filled with no, NO2 and O2 mixture upside down in the water tank, completely dissolve it, there is no gas remaining, and the product does not diffuse, then the value range of material concentration (c) of the obtained solution is ()

（A） （B）

（C） （D）

3. When m mol Cu reacts with a certain amount of concentrated HNO3, NL gas can be generated under standard conditions, the most likely relationship between the values of M and N is ()

(A) (b) (c) (d) unable to judge

4. React a certain mass of Mg, Zn and Al mixture with a sufficient amount of dilute H2SO4 to produce H2 2.8 L (standard condition). The mass of the original mixture may be ()

A. 2g B. 4g C. 8g D. 10g

Detailed answer to calculation method:

1、 1. The mass reduction of the analytical mixture is caused by the decomposition of sodium bicarbonate. The solid mass difference is 21.0g-14.8g = 6.2g, that is, the mass of CO2 and H2O generated. M (NaHCO3) = 168 in the mixture × 6.2g ÷ 62 = 16.8g, m (Na2CO3) = 21.0g-16.8g = 4.2g, so the mass fraction of sodium carbonate in the mixture is 20%

2. Analysis this example is a calculation problem involving solubility. To solve this problem, you should have the ability to understand the concept thoroughly, find the actual difference and complete simple calculation. It is told that at 100 ℃ and 30 ℃, K can be dissolved in 100g water at most

### Calculation problem The solid mixture of sodium carbonate, sodium bicarbonate and NaCl is 8g in total. Strengthen the heat until it is no longer reduced. After cooling, weigh it to 6.76g. Add excess hydrochloric acid to the residual solid to produce 892ml of carbon dioxide (density 1.977g / L). Calculate the mass of sodium carbonate, sodium bicarbonate and NaCl in the mixture respectively

Among the three substances, only sodium bicarbonate will decompose when heated. The mass reduction of 2nahco3 = Na2CO3 + H2O + CO2 is 18 + 44 = 62. Now the weight loss after heating is 8-6.76 = 1.24, so the original amount of NaHCO3 is 168 × 1.24 / 62 = 3.36g, and the mass of Na2CO3 is 106 × 3.36 / 168 = 2.12g salt

### After the mixture of 12.4g Fe2O3 and CuO fully reacts with sufficient Co, the solid decreases by 3.1g, and the mass ratio of Fe2O3 and CuO in the original mixture is () A. 1：1 B. 2：1 C. 3：1 D. 3：2

Let the mass of Fe2O3 in the mixture of 12.4g Fe2O3 and CuO be x, then the mass of CuO is 12.4g-xfe2o3 + 3Co high temperature . 2Fe+3CO2160 one hundred and twelve x ...