What is the 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 at all so complicated. Do you still have difficulties in such tasks? Then luck smiled at you, you found this article! Interesting? Then read more, now you will understand everything.
What is mass fraction?
So, for starters, find out what a mass fraction is. Any chemist will answer how to find the mass fraction of an element in a substance, since they often use this term when solving problems or while in the laboratory. Of course, because her calculation is their daily task. To obtain a certain amount of a 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. Therefore, this topic is so important.
This term is indicated by the symbol “w” and reads as “omega”. It expresses the ratio of the mass of a given substance to the total mass of a mixture, solution or molecule, expressed as a fraction or as a percentage. The formula for calculating the mass fraction:
w = m substance / m mixture .
Transform the formula.
We know that m = n * M, where m is 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 units of mass or a. E. m. Such a unit of measurement is equal to one twelfth fraction of the mass of the carbon core 12. The value of the molecular mass can be found in the periodic table.
The amount of substance n of the desired object in this mixture is equal to the index times 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.
You should not be afraid of such cumbersome definitions or formulas, a certain logic can be traced in them, having understood which, even the formulas themselves can not be taught. The molar mass M is equal to the sum of the atomic masses A r of a given substance. Recall that atomic mass is the mass of 1 atom of a substance. That is, the original formula for the mass fraction:
w = (n substances * M substances ) / m mixture .
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 coincide. Although the mixture a priori cannot consist of one substance.
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.
Verification of the acquired material. Easy task
Now we will analyze two tasks: easy and intermediate. Read on!
It is necessary to find out the mass fraction of iron in the molecule of iron sulfate FeSO 4 * 7 H 2 O. How to solve this problem? Consider the solution below.
Decision:
Take 1 mol of FeSO 4 * 7 H 2 O, then we find out the amount of iron by multiplying the coefficient of iron by its index: 1 * 1 = 1. 1 mole of iron is given. 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. E. m. = 56 grams / mol. In this case, A r = M. Therefore, m iron = n * M = 1 mol * 56 gram / mol = 56 g.
Now you need to find the mass of the whole molecule. It is equal to the sum of the masses of the starting materials, i.e. 7 mol of water and 1 mol of iron sulfate.
m = (n water * M water ) + (n iron sulfate * M iron sulfate ) = (7 mol * (1 * 2 + 16) gram / mol) + (1 mol * (1 mol * 56 gram / mol + 1 mol * 32 grams / mol + 4 mol * 16 grams / mol) = 126 + 152 = 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%.
Mid-level challenge
We solve a more complex problem. 34 g of calcium nitrate are dissolved in 500 g of water. It is necessary to find the mass fraction of oxygen in the resulting solution.
Decision
Since during the interaction of Ca (NO 3 ) 2 with water, only the dissolution process takes place, and reaction products do not stand out from the solution, the mass of the mixture is equal to the sum of the masses of calcium nitrate and water.
We need to find the mass fraction of oxygen in the solution. We draw attention to the fact that oxygen is contained both in the dissolved substance and in the solvent. Find the amount of the element in water. To do this, calculate the mole of water according to the formula n = m / M.
n water = 500 g / (1 * 2 + 16) gram / mol = 27.7777≈28 mol
From the water formula H 2 O we find that the amount of oxygen = the amount of water, that is, 28 mol.
Now we find the amount of oxygen in dissolved Ca (NO 3 ) 2 . To do this, we find out the amount of the substance itself:
n Ca (NO3) 2 = 34 g / (40 * 1 + 2 * (14 + 16 * 3)) gram / mol ≈ 0.2 mol.
n Ca (NO3) 2 refers to n O as 1 to 6, which follows from the formula of the compound. Therefore, n O = 0.2 mol * 6 = 1.2 mol. The total amount of oxygen is 1.2 mol + 28 mol = 29.2 mol
m O = 29.2 mol * 16 gram / mol = 467.2 g.
m solution = m water + m Ca (NO3) 2 = 500 g + 34 g = 534 g
It remains only to calculate the mass fraction of a chemical element in a substance:
w O = 467.2 g / 534 g≈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 it is well understood. We wish you good luck and success in future endeavors.