chem modelsofparticulatenatureofmatter
1.4.4 the empirical formula of a compound gives the simplest ratio of atoms of each element present in that compound. the molecular formula gives the actual number of atoms of each element present in a molecule.
empirical formula is the simplest whole-number ratio of the atoms of each element in a compound
the formulas of all ionic compounds are empirical formulas
anhydrous salt + water of crystallisation = hydrated salt
converting percentage by mass to empirical formulas
- divide the percentage mass of each element by its molar mass,
, to convert it to moles - divide the moles of each element by the smallest number to give the ratio of moles
- approximate to the nearest whole number
converting empirical formulas to percentage by mass
i.e. what is the percentage by mass of
the molecular formula of a compound gives the actual numbers of atoms in a molecule. formulas that show all the atoms present in a molecule are called molecular formulas
challenge questions
- A fertiliser has an N-P-K rating of 18-51-20. Use the information in the box above to determine the percentage by mass of nitrogen, phosphorus and potassium present.
%N in fertiliser is 18%
%P in fertiliser is 22.3%
%K in fertiliser is 16.6%
interesting
A 0.5438g sample of a compound known to contain only carbon, hydrogen, and oxygen was burned completely in oxygen. The products were 1.0390g
| C | H | O | |
|---|---|---|---|
| mole | 0.02361 | 0.07076 | 0.01186 |
| ratio | 1.99 | 5.97 | 1 |
| nearest | 2 | 6 | 1 |
thus, the molecular formula is the same as the empirical formula, which is