What is mass fraction in chemistry? Do you know the answer? How to find the mass fraction of an element in a substance? The calculation process itself is not so complicated at all. Are you still having trouble doing this kind of work? Then luck smiled at you, you found this article! Interesting? Then read on, now you will understand everything.
What is mass fraction?
So, first, let's find out what a mass fraction is. How to find the mass fraction of an element in a substance, any chemist will answer, since they often use this term when solving problems or during their stay in the laboratory. Of course, because its calculation is their daily task. To obtain a certain amount of a particular substance in laboratory conditions, where accurate calculation and all possible outcomes of reactions are very important, you need to know just a couple of simple formulas and understand the essence of the mass fraction. That is why this topic is so important.
This term is denoted by the symbol “w” and is read as “omega”. It expresses the ratio of the mass of a givensubstance to the total mass of a mixture, solution or molecule, expressed as a fraction or as a percentage. Mass fraction formula:
w =m substances / m mixtures.
Transform the formula.
We know that m=nM, where m is the mass; n is the amount of substance, expressed in units of mole; M is the molar mass of the substance, expressed in grams/mol. The molar mass is numerically equal to the molecular mass. Only molecular weight is measured in atomic mass units or a. e. m. Such a unit of measurement is equal to one twelfth of the mass of the carbon nucleus 12. The value of the molecular weight can be found in the periodic table.
The amount of substance n of the desired object in a given mixture is equal to the index multiplied by the coefficient for this compound, which is very logical. For example, to calculate the number of atoms in a molecule, you need to find out how many atoms of the desired substance are in 1 molecule=index, and multiply this number by the number of molecules=coefficient.
Don't be afraid of such cumbersome definitions or formulas, they trace a certain logic, understanding which, you can not even learn the formulas themselves. The molar mass M is equal to the sum of the atomic masses Ar of the given substance. Recall that atomic mass is the mass of 1 atom of a substance. That is, the original mass fraction formula:
w =(n substancesM substances)/m mixtures.
From this we can conclude that if the mixture consists of one substance, the mass fraction of which must be calculated, then w=1, since the mass of the mixture and the mass of the substance are the same. Although a mixture a priori cannot consist of onesubstances.
So, we figured out the theory, but how to find the mass fraction of an element in a substance in practice? Now we will show and tell everything.
Checking learned material. Easy Challenge
Now we will analyze two tasks: easy and medium level. Read more!
It is necessary to find out the mass fraction of iron in the molecule of ferrous sulfate FeSO47 H2O. How to solve this problem? Consider the solution further.
Solution:
Take 1 mol of FeSO47 H2O, then find out the amount of iron by multiplying the coefficient of iron by its index: 1 1=1. Given 1 mole of iron. We find out its mass in matter: from the value in the periodic table, it can be seen that the atomic mass of iron is 56 a.u. e.m.=56 grams / mol. In this case, Ar=M. Therefore, m iron \u003d nM \u003d 1 mol56 grams / mol \u003d 56 g.
Now you need to find the mass of the entire molecule. It is equal to the sum of the masses of the starting substances, that is, 7 mol of water and 1 mol of ferrous sulfate.
m=(n water M water) + (n ferrous sulfate M iron sulfate)=(7 mol(12+16) gram/mol) + (1 mol (1 mol56 gram/mol+1 mol32 gram/ mol + 4 mol16 grams / mol) u003d 126 + 152 \u003d 278 g.
It remains only to divide the mass of iron by the mass of the compound:
w=56g/278g=0.20143885~0.2=20%.
Answer: 20%.
Intermediate level task
Let's solve a more difficult problem. 34 g of calcium nitrate are dissolved in 500 g of water. You need to find the mass fraction of oxygen in the resulting solution.
Decision
Soas in the interaction of Ca(NO3)2 with water, only the dissolution process occurs, and no reaction products are released from the solution, the mass of the mixture is equal to the sum of the masses of nitrate calcium and water.
We need to find the mass fraction of oxygen in the solution. Note that oxygen is contained in both the solute and the solvent. Find the amount of the desired element in water. To do this, we calculate the mole of water according to the formula n=m/M.
n water=500 g/(12+16) gram/mol=27.7777≈28 mol
From the formula of water H2O we find that the amount of oxygen=the amount of water, that is, 28 mol.
Now find the amount of oxygen in dissolved Ca(NO3)2. To do this, we find out the amount of the substance itself:
n Ca(NO3)2=34 g/(401+2(14+163)) gram/mol≈0.2 mol.
n Ca(NO3)2 refers to n O as 1 to 6, as follows from the compound formula. So, n O=0.2 mol6=1.2 mol. The total amount of oxygen is 1.2 mol+28 mol=29.2 mol
m O=29.2 mol16 gram/mol=467.2 g
m solution=m water + m Ca(NO3)2=500g +34 g=534 g.
It remains only the calculation of the mass fraction of a chemical element in a substance:
w O=467.2g /534g≈0.87=87%.
Answer: 87%.
We hope that we have clearly explained to you how to find the mass fraction of an element in a substance. This topic is not at all difficult if you understand it well. wishgood luck and success in your future endeavors.
