To illustrate, consider the well-known all carbon, without which it would not be organic.If carbon represents a pure substance (eg, diamond), its mass share can safely take per unit or 100%.Naturally, the diamond also contains impurities of other elements, but in most cases in such small amounts that they can be neglected.But such modifications of carbon as coal or graphite, the impurity content is quite high, and this neglect is unacceptable.
If the carbon is part of a complex matter, it is necessary to proceed as follows: write down the exact formula of substance, then, knowing the molar mass of each element , included in its composition, to calculate the exact molar mass of the substance (Of course, taking into account the "index" of each element ).Then determine the mass share , dividing the total molar mass element to the molar mass of the substance.
example, need to find a mass share carbon in acetic acid.Write a formula of acetic acid: CH3COOH.To facilitate the calculations, convert it into a form S2N4O2.The molar mass of the substance consists of the molar masses of the elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this material is calculated as follows: 24/60 = 0.4.
If you calculate it as a percentage of, respectively, 0.4 * 100 = 40%.That is, for each kilogram of acetic acid contained (approximately) 400 grams of carbon.
course, quite the same way you can find the mass fractions of all other elements.For example, the mass fraction of oxygen in the same acetic acid is calculated as follows: 32/60 = 0.533, or approximately 53.3%;and the mass fraction of hydrogen is 4/60 = 0.666, or about 6.7%.
to check the accuracy of the calculations add up the percentages of all the elements: 40% (carbon) + 53.3% (oxygen) + 6.7% (hydrogen) = 100%.Account converged.