To define a mass fraction of an element, define a chemical formula of substance which mass fractions of elements it is calculated. By means of periodic Mendeleev's system determine atomic masses of all elements and molecular mass of the studied substance. Then find the relation of atomic mass of this element to molecular weight.
It is required to you
- periodic table of Mendeleyev, calculator.
Instruction
1. Define a chemical form of substance which mass fractions of elements need to be found. Take periodic Mendeleev's system and find in it the cells of elements corresponding to the atoms which are a part of a molecule of this substance. Find mass number of each such element in a cell. If the found value of mass number of an element fractional, round it to the next whole.
2. When atoms of one type meets in a molecule several times, increase their atomic mass by this number. Slozhimte of the mass of all elements which are a part of a molecule to receive value in atomic units of mass. For example, if it is necessary to find the mass of a molecule of salt which is called sulfate of sodium (Na2SO4), determines the atomic mass of Ar (Na) =23 sodium, Ar (S) =32 sulfur and Ar (O) oxygen =16. As the molecule contains 2 atoms of sodium, for it take value 23*2=46, and for oxygen which has 4 atoms - 16*4=64. Then the mass of a molecule will be sulfate of sodium will be Мr(Na2SO4)=46+32+64=142.
3. To count mass fractions of the elements which are a part of a molecule of this substance find the relations of mass of the atoms entering a substance molecule to the mass of the molecule, and increase result by 100%. For example, if to consider Na2SO4 sodium sulfate, calculate mass fractions of its elements thus: - the mass fraction of sodium will make ω(Na) = 23•2•100%/142=32.4%;-the mass fraction of sulfur will make ω(S) = 32•100%/142=22.5%;-the mass fraction of oxygen will make ω (O) = 16•4•100%/142=45.1%.
4. Mass fractions show the relative maintenance of elements in this molecule of substance. Check correctness of calculation, having put mass fractions of substance. Their sum has to be 100%. In the reviewed example of 32.4%+22.5%+45.1%=100%, calculation means it is made correctly.