There are many "equivalences" in chemical knowledge A. The total mass of the substance participating in the chemical reaction is equal to the total mass of the substance formed after the reaction. B. the total number of positive and negative valence of the elements in the compound is equal. C. the mass of solute before and after dilution is equal. D. the number of protons in the nucleus of the atom is equal to the number of neutrons

A. According to the law of conservation of mass, in a chemical reaction, the sum of the masses of the substances before the reaction is equal to the sum of the masses of the substances after the reaction. Therefore, a is correct; B, because the compound is not externally charged, the total number of positive valence and negative valence of the elements in the compound is equal. Therefore, B is correct; C, the solution is dilute

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A chemical fertilizer plant needs a large amount of CO2 to produce chemical fertilizer. If it wants to buy a batch of limestone as soon as possible (the impurities do not participate in any reaction), the manufacturer will analyze and determine the price at the place of origin. They take 2G and add 20g dilute hydrochloric acid in four times. After fully reflecting, the quality of each remaining solid is shown in the table below;
The results are as follows: the concentration of the hydrochloric acid in the hydrochloric acid is: the concentration of \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\themass of the remaining solid
It is the first time to add 5g, and it will be the first time to add 5g, and it will be the first time to add 5g and 35; # 35\\35\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\65371; 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; ｛ 160; 1.4g
In the second second time, we added 5g, which was the first time to add 5g, and it was the first time to add 5g, and it was also the first time to add 5g, and it was also the first time to add 5g, and it was 35;35;; 35;35;35;35;35;35;\35\35\35\\ \\\\\\\\\\\\\\\\ ; 160;;;;; 160;;;;;;;;;;;;;;;;;;;;;; 160;;;;;;;;;;;;; # 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; 0.8g
In the third third time, we added 5g to the third time to add 5g, which was the first time to add 5g, which was the first time to add 5g 35;; 35;35;35;35;35;#35;35;35;3535#35;\35\ \\\\\35\ \\\\\ ;;;;;; 160;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; \\\\\\\35; 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; 0.4g
In the fourth time, we added 5g to the fourth time to add 5g, which was the first time to add 5g, which was also the first time to add 5g, and it was also the first time to add 5g, and it was also the first time to add 5g, and it was also 160; and it was also 160; and it was also 160; and it was also 160; and it was also the first time to add 5g, and it was also \35\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; &# 160; 0.4g
Try to calculate: (1) the mass of impurities in 2G limestone sample is - (?) 160; (2) the mass fraction of CaCO 3 in limestone; (3) the mass fraction of CO 2 can be obtained after 1000 kg limestone is fully calcined; (40) the mass fraction of HCl in dilute hydrochloric acid used?

(1) The mass of impurity in 2G limestone sample is 0.4g. (2) what is the mass fraction of CaCO3 in limestone? (2g-0.4g) / 2G * 100% = 80% (3) what is the mass of CO2 after full calcination of 1000kg limestone? 1000kg * 80% = 800kg CaCO3 = CaO + CO2 100

The mathematical problems start at the same time and travel in opposite directions. When the passenger cars meet, they travel 36 kilometers more than the freight cars
Passenger and freight cars start from both places at the same time and travel in opposite directions. When they meet, the passenger car travels 36 kilometers more than the freight car. Given that the speed ratio of passenger and freight cars is 9:7, it takes 3 hours for the passenger car to complete the whole journey. Please find out how many kilometers the passenger car travels per hour

Thinking: first of all, we need to know the formula: distance = speed times time, expressed by the formula is s = VT, then, because the bus and truck start at the same time, they travel the same time when they meet. From the formula s = VT, we can see that since the speed ratio of the two vehicles is 9:7, the distance of the two vehicles is 9:7 when they take the same time t
Because the speed ratio of the two vehicles is 9:7 and the driving time is equal, according to s = VT, the distance ratio of the two vehicles is also 9:7
If the passenger car runs 9x km and the freight car runs 7x km, the distance between a and B is 9x + 7x = 16x km
9x－7x＝36
x＝18
Then 16x = 288
V (bus) = s / T = 288km / 3H = 96km / h
A: the bus runs 96 kilometers per hour

What is projection?

Let's go through the math books. The following can be referred to: a projection makes a vertical line from a point to a straight line or a plane, and the resulting perpendicular foot is the projection of this point on the straight line or plane; a projection is a figure, such as the projection of a point or a line segment on a plane in geometry, which generally refers to its perpendicular foot or the line segment between perpendicular feet, etc, It can be obtained by using it as a method to find the perpendicular foot

The two cars run from a and B at the same time. The bus runs 54km per hour and the truck 45km per hour. When they meet, the bus runs 36km more than the truck. How many km is the distance between a and B?

It took two cars x hours
Then 54x-45x = 36
The solution is x = 4
So the distance between a and B is 54 times 4 plus 45 times 4 = 396km

Simplify the square of (a-b) + (B-C) + (C-A)

Square of (a-b) + square of (B-C) + square of (C-A)
=2 * A's square + 2 * B's square + 2 * C's Square - 2 * a * B-2 * a * C-2 * b * C
Just open it!

Li Wei rides a motorcycle from his home to the railway station. If he travels 30 kilometers per hour, he will travel 15 minutes earlier than the train,
If Li Wei wants to arrive at the railway station 10 minutes before the train leaves, what's the speed of his motorcycle?
Linear equation of one variable

Setting: Li Weiyuan planned to spend X minutes
30 km / h = 0.5 km / min; 18 km / h = 0.3 km / min
0.5（X-15）=0.3（X+15）,
X = 60 minutes (original plan)
Then: the actual speed is y km / min
0.5（60-15）=Y（60-10）
Y = 0.45 km / min = 27 km / h
Hehe, can we set two unknowns?

Given a = a + 2, B = a2-a + 5, C = A2 + 5a-19, where a > 2, find B-A >, and point out the size of a and B

B-A
=a2-a+5-a-2
=a2-2a+3
=(a-1)^2+2
Because a > 2
So (A-1) ^ 2 + 2 > 3 is B-A > 3
Because B-A > 3 > 0, B is larger

A car uphill and downhill round-trip distance of 420 km, the car uphill speed of 30 km
The speed down the mountain is 42 km. Find the average speed of the car up and down the mountain

Third grade questions?
(uphill speed + downhill speed) / 2 = average speed
Uphill speed 36 km / h
Downhill speed 42 km / h
All the conditions are known, is not the third grade problem?
(30 + 42) / 2 = 36 km

It is known that: x = 2a-3, y = 4A + 3, y is expressed by X, and Y is obtained=______ ．

By multiplying the first equation by 2 and subtracting the second equation, we get: 2x-y = - 6-3, - y = - 2x-9, y = 2x + 9