Mass fraction refers to the ratio of solute mass in solution to solution mass, and also refers to the percentage of certain substance mass in mixture to total mass[5]。The mass fraction can also refer to the ratio of the relative atomic mass of each atom in the compound (coefficient multiplication required) to the total formula weight, that is, the proportion of an element in a substance[1]。
Mass fraction refers to the percentage of solute mass and solution mass (the fraction of solute B mass in solution mass) or expressed by the symbol ω.
The calculation formula is:
In the above formula, the dimension of ω (B) is 1, which can also be expressed as a percentage.[2]
Definition II
The mass fraction can also refer to the ratio of the relative atomic mass of each atom in the compound (coefficient multiplication required) to the total formula weight, that is, the proportion of an element in a substance.E.g. substance FetwoOthree（ferric oxide）Then the mass fraction of Fe is 56 * 2/160 * 100%=70%.The mass fraction of elements has a wide range of uses, among which the "nutrient composition" label on food packaging requires the mass fraction of elements.
Conceptual discrimination
Announce
edit
Solubility and saturation
one
Solubility is used to indicate the solubility of a substance in a solvent at a certain temperature.The mass fraction of solute is used to indicate the composition of solution.
two
Concentrated solutionThe mass fraction of medium solute is large, but not necessarily saturated solution, and the mass fraction of solute in dilute solution is small, but not necessarily unsaturated solution.
Calculation formula of mass fraction
Calculation of mass fraction of saturated solution:
matters needing attention
one
soluteThe mass fraction of only represents the solute mass andsolutionThe ratio of mass does not represent the specific solution mass and solute mass[3]。
two
The unit of solute mass and solution mass in the calculation formula of solute mass fraction must be unified[3]。
three
If all solutes are dissolved inwater, and does not occur with waterchemical reaction, using the calculation formula directly.In the formula, the solute mass refers to the mass of the dissolved part of solute, and the mass of the undissolved part of solute cannot be included[3]。
four
If the solute isCrystalline hydrateAfter dissolved in water, the quality of its solute does not include the quality of crystal water.Because when the crystal hydrate is dissolved in water, the crystal water will be converted into solvent in the solution, and the quality of water will increase[3]。
five
aboutacid、alkali、Salt solutionIntermittent 1~2chemical reactionCalculate the solution solute mass fraction problem after reaction.First, it is necessary to determine what the solute in the generated solution is, then calculate the solute mass through chemical reaction, (usually the solute mass is composed of several parts), and finally analyze the relationship between the quantities to obtain the total solutionqualityAnd then calculate the mass fraction of solute in the solution after reaction[4]。a、Solution composition method
Solution mass=solute mass+solvent mass, in which solute must be dissolved, and solvent water usually has two situations according to different topics: ① water in the original solution, ② water newly generated by chemical reaction.
b、Mass conservation method
Solution mass=m (reaction) - m ↓ - m ↑. This method is relatively simple. m (reaction) represents the total mass of reactants (or solutions), m ↓ represents the mass of insoluble solids and precipitates generated, and m ↑ represents the mass of gas generated.
Dilution enrichment
Announce
edit
dilution
(1) Add waterdilution: Because the mass of solute remains unchanged before and after the solution is diluted, if the mass of concentrated solution is A gram, the mass fraction of solute is a%, and B grams of water is added to dilute the solution into C grams of dilute solution with the mass fraction of solute being b%.Calculation formula
⑵ Dilute with dilute solution
Because the total mass of the mixed solution is equal to the sum of the mass of the two mixed component solutions, the mass of the solute in the mixed solution is equal to the sum of the mass of the solute in the two mixed components.Therefore, if the mass of the original solution is A g, the mass fraction of the solute is a%, the mass fraction of the dilute solution is B g, and the mass fraction of the solute is b%, and the two solutions are mixed to obtain a solution with the mass fraction of the solute of c%, then:
The mass of the solvent remains unchanged before and after the solution increases the solute.After adding solute, the mass of solute in solution=the mass of solute in original solution+the mass of added solute, while the mass of solution=the mass of original solution+the mass of added solute.
Therefore, if the original solution mass is A g, the solute mass fraction is a%, and the solute Bg is added, it will become a solution with the solute mass fraction of b%.
(2) Evaporation
Because the mass of solute remains unchanged before and after solvent evaporation.Therefore, if the original solution mass is A g, the solute mass fraction is a%, and Bg water is evaporated into a solution with a solute mass fraction of b%, then: Ag × a%=(Ag － Bg) × b%.
(3) Mixed with concentrated solution
Because the total mass of the mixed solution is equal to the sum of the mass of the two mixed component solutions, the mass of the solute in the mixed solution is equal to the sum of the mass of the solute in the two mixed components.Therefore, if the mass of the original solution is A g, the mass fraction of the solute is a%, the mass fraction of the concentrated solution is B g, and the mass fraction of the solute is b%, and the two solutions are mixed to obtain a solution with the mass fraction of the solute of c%, then:
C * M=1000 * ρ * ω is the quantity concentration of the substance multiplied byMolar massIt is equal to 1000 times the density times the mass fraction.
