Mass fraction? And which one?
In the process of chemical reaction, it is not alwayswe get what is theoretically planned, at least quantitatively. Usually this is due to the difficult reaction conditions - non-ideal temperature, insufficient contact with the catalyst and just chemical impurities of the reagents. In this case, chemists use the phrase "mass yield share".
This concept includes a specific value -the percentage of practically obtained in relation to what would have been chemically produced. Denoted by the letter "omega". This value should be taken into account, often students forget to recalculate a small percentage. Particularly offensive is on all sorts of tests - the course of thought is correct, and the standard exam would allow most of the score for the task to be counted - and in testing it is on such little things that they "catch". Even give the answer options given this error. It's easy to get caught. So before you solve the problem, check whether there is a parameter "Mass fraction of output".
There are other, similar sounding concepts. The term "mass fraction" itself can be combined with other terms. And then it turns out, for example, the proportion of matter in the ore. That is, you have a piece of material in which only some part can react. And this need to be taken into account in the calculations, otherwise you risk falling into the trap, as with the notion of "mass exit share." Also catch successfully and many. Caution!
In the condition there is a mass fraction of the element inconnection? This means that its atoms constitute a certain proportion of the mass in the substance. In principle, the chemists and fans of complex solutions can use the mass fraction to calculate the reaction equations. Another practical value of this information can be if you need to establish a formula of the substance. Just be careful - there are substance-isomers and substances with a similar proportional formula. You will need chemical reactions to establish the exact formula. But this level is not a school, but an olympiad in chemistry.
In reality, usually, all tasks are mucheasier, school children are tested for knowledge of the elementary formula and the ability to produce simple mathematical operations, without forgetting the number of atoms per molecule. How is the mass fraction of the element calculated? From the table, find the atomic weight of the desired element, multiply by the exact number of atoms in the molecule. This is the numerator. And in the denominator there must be a molecular mass of a unit of substance of the whole formula, that is, your element with the necessary factors and all other masses of elements multiplied by their number in the molecule. For example, the molecular weight of the water molecule is 16 (oxygen), add two hydrogen atoms (1 + 1). Total 18. The mass fraction of the hydrogen element is simple: divide 2 by 18. If necessary, multiply by one hundred percent, but in fractions of a unit, you can also. By analogy, do in more complex formulas, when the elements are three or more.
If you are careful and understand each of thepossible cases, you will not be slapped on the elementary. And will not be offended because of a low score, when everything seems to be decided, but the result is not happy. Just do not miss these terms out of sight. Study and practice on specific tasks. When you type your hand, all the difficulties will be in the past.