You will need
  • - the periodic table;
  • calculator.
Translate the grams into moles by the formula:
n is the number of moles you want to find
m is the known mass of substance (g)
M - the mass of one mole of a substance, or molar mass (g/mol)
Thus, to solve the problem remains to know the value of M.
Molar mass is the value for each substance is constant, and in the General case, is numerically equal to its relative atomic or molecular mass, relative. To determine the molecular or atomic composition of the analyte, it is necessary to look at its chemical formula. For example, H2O (water) is a molecule of O2 (oxygen) molecule, Fe (iron) atom, C (carbon) - atom.
For atomic matter, it is sufficient to find it in the periodic table - relative atomic mass is specified in the cell of each item. For example, the relative atomic mass of substances, Fe, Na = 12, 56, 23 (rounded to integer) - hence, their molar mass M is 12 g/mol 56 g/mol 23 g/mol.
If the substance is molecular, then the relative molecular mass is the sum of the atomic masses of all the atoms within the molecule. For example, the relative molecular mass of water with formula H2O is 18 - two atoms of hydrogen with mass of 1 to fold with one oxygen atom with a mass 16 (2*1+16=18). The methane molecule - CH4 - relative molecular mass will be 16 (12+4*1=16). Thus, the molar masses M of water and methane will equal 18 g/mol and 16 g/mol respectively.
Now, knowing the mass of the substance m in grams and the molar mass M, obtained with the help of the periodic table and simple calculations, converting grams to moles according to the above formula: n=m/M. the Number n is the required amount of moles for the given grams of substance.