How to define a mass fraction of substance

How to define a mass fraction of substance

The mass fraction of substance shows its keeping in more complex structure, for example, in alloy or mix. If the lump of mix or alloy is known, then knowing mass fractions of the making substances it is possible to find their masses. To find a mass fraction of substance, it is possible knowing its mass and mass of all mix. This size, can be expressed in submultiple sizes or percent.

It is required to you

  • scales;
  • periodic table of chemical elements;


1. Define a mass fraction of substance which is in mix through the mass of mix and the substance. For this purpose determine the mass of the substances making mix or alloy by scales. Then put them. Take the received weight for 100%. To find a mass fraction of substance in mix, divide its mass of m into the mass of mix M, and result increase by 100% (ω %= (m/M) of ∙100%). For example, in 140 g of water dissolve 20 g of table salt. To find a mass fraction of salt, put the mass of these two substances M=140+20=160 of. Then find a mass fraction of substance ω %= (20/160) ∙100%=12.5%.

2. If it is required to find the content or a mass fraction of an element in substance with the known formula, use the periodic table of chemical elements. On it find atomic masses of elements which are a part of substance. If one element meets in a formula several times, increase its atomic mass by this number and put the received results. It will be the molecular mass of substance. To find a mass fraction of any element in such substance, divide its mass number in this chemical formula M0 into the molecular mass of this substance M. Result multiply for 100% (ω %= (M0/M) of ∙100%).

3. For example, define a mass fraction of chemical elements in copper vitriol. Copper vitriol (copper II sulfate), has chemical formula CuSO4. Atomic masses of the elements which are its part are equal to Ar (Cu) =64, Ar (S) =32, Ar (O) =16, mass numbers of these elements will be equal to M0 (Cu) =64, M0(S)=32, M0(O)=16∙4=64 taking into account that the molecule contains 4 atoms of oxygen. Calculate the molecular mass of substance, it is equal to the sum of mass numbers of substances 64+32+64=160 making a molecule. Define a mass fraction of copper (Cu) as a part of copper vitriol (ω %= (64/160) ∙100%) =40%. It is possible to determine mass fractions of all elements in this substance by the same principle. Mass fraction of sulfur (S) ω %= (32/160) ∙100%=20%, oxygen (O) ω %= (64/160) ∙100%=40%. Pay attention that the sum of all mass fractions of substance has to be 100%.

Author: «MirrorInfo» Dream Team