Example At room temperature, 1 volume of ethylene is mixed with a certain amount of gaseous unknown hydrocarbon, the relative density of mixed gas and hydrogen is measured as 12, and the volume of the hydrocarbon is calculated.
According to the calculation of relative density, the average molecular formula of mixed gas is 24, and the molecular formula of ethylene is 28, so the molecular formula of unknown hydrocarbons must be less than 24, and the hydrocarbons less than 24 are only methane. By using the cross method, we can find that the volume of methane is 0.5.
(2) Cross method for calculating the atomic percentage of isotopes.
There are two isotopes of bromine. In nature, these two isotopes account for about half. It is known that bromine has an atomic number of 35 and an atomic weight of 80, so the neutron number of two isotopes of bromine is equal to.
79
、8 1
45 years old
、46
44
、45
44
、46
According to the cross method, the difference between the atomic weights of two isotopes and bromine is equal, so their neutron numbers should be different by 2, so the answer is D.
(3) Cross method in solution preparation calculation
Example A Students should prepare100g of 40% NaOH solution, and there are 10% NaOH solution and NaOH solids in the laboratory. How many grams of each of the above substances should this student take?
The analysis shows that the solute of 10%NaOH solution is 10, the solid solute of NaOH is 100, and the solute of 40%NaOH solution is 40. The required 10% NaOH solution was obtained by orthogonal method.
×100 = 66.7g, and the solid is sodium hydroxide.
× 100 = 33.3g。
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4) Cross method in calculation of mixture reaction.
For example,100g mixture of lithium carbonate and barium carbonate consumes the same amount of hydrochloric acid when they react with hydrochloric acid of a certain concentration as when100g calcium carbonate reacts with hydrochloric acid of this concentration. Calculate the molar ratio of lithium carbonate to barium carbonate in the mixture.
The formula amount of calcium carbonate can be understood as the average formula amount of the mixture of lithium carbonate and barium carbonate, and the ratio of lithium carbonate to barium carbonate can be calculated by cross method, which is 97:26.