The quantity of molecules cannot almost be measured in substance by usual methods. It is connected with the fact that the molecule of substance is too small in order that to see it. Therefore the quantity of molecules in this mass of substance pays off with the help of special formulas.
It is required to you
- - periodic table of chemical elements;
- - scales;
- - calculator.
Instruction
1. Knowing such size as amount of substance ν, find number of molecules in it. For this purpose increase the amount of substance measured in moths by constant Avogadro (NA =6,022∙10^23 1/mol) who is equal to number of molecules in 1 mole of a substance of N=ν/NA. For example, if there are 1.2 mol of table salt, then it contains N=1,2∙6,022∙10^23 ≈7.2∙10^23 molecules.
2. If the chemical formula of substance is known, by means of the periodic table of elements find its molar weight. For this purpose according to the table find relative atomic masses of atoms of which the molecule consists, and put them. As a result receive the relative molecular mass of substance which is in number equal to its molar weight in grams on mol. Then, on scales measure the mass of the studied substance in grams. To find quantity of molecules in substance, increase the mass of substance m by constant Avogadro (NA =6,022∙10^23 1/mol) and divide result into the molar mass of M (N=m ∙ NA/M).
3. An example Define quantity of molecules which contains in 147 g of sulfuric acid. Find the molar mass of sulfuric acid. Its molecule consists of 2 atoms of hydrogen of one atom of sulfur and 4 atoms of oxygen. Their atomic masses are equal to 1, 32 and 16. Relative molecular weight is equal to 2∙1+32+4∙16=98. It is equal to molar weight therefore g/mol M=98. Then the quantity of the molecules which are contained in 147 g of sulfuric acid will be equal to N=147∙6.022∙10^23/98≈9∙10^23 of molecules.
4. To find quantity of molecules of gas in normal conditions at a temperature 0ºС and pressure of 760 mm rt. a column, find its volume. For this purpose measure or calculate the volume of capacity of V in which it is in liters. To find quantity of molecules of gas divide this volume into 22.4 l (the volume of one of gas in normal conditions), and increase by Avogadro's (NA =6,022∙10^23 1/mol) N number = V ∙ NA/22.4.