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OCR A-level Chemistry Formulae

Formulae

This page covers the following topics:

The molecular formula denotes all the atoms of each element present in a compound. The empirical formula represents the smallest ratio of elements in a compound.

To get an empirical formula from the molecular formula, find the highest common factor of all the coefficients in molecular formula, including the hidden β€œ1” (when there is just a single atom of some element in a compound) and divide all of the coefficients in the formula by this greatest common factor.

Empirical & molecular formulae

There are many compounds that are regularly used in chemistry, so their chemical formula and name are presumed to be common knowledge. Some of them are provided in the list below:

Hβ‚‚O is water,
NH₃ - ammonia,
CHβ‚„ - methane,
COβ‚‚ - carbon dioxide,
SiOβ‚‚ - sand.

Formulae of common compounds

An ion is an atom or a molecule which has either a positive or negative charge, due to the loss or gain of electrons. There are many ions that are regularly used in chemistry, thus their chemical formula, charge and name are presumed to be common knowledge. Some of them are provided in the list below:

OH⁻ is hydroxide,
CO₃²⁻ - carbonate,
NO₃⁻ - nitrate,
PO₄³⁻ - phosphate,
SO₄²⁻ - sulphate,
SO₃²⁻ - sulphite,
NH₄⁺ - ammonium ion.

Ions

You can predict the formulae of ionic salts by balancing the charges of the anion and cation. The name is then derived by combining the names of the two ions, using a suitable suffix for the anion (eg. -ide, -ate). When predicting the formulae of covalent molecules, the valency of each atom must be considered. The number of bonds an atom can make depends on the number of unpaired electrons present in the outer shell. Each atom in the compound must make the correct number of covalent bonds. Remember, some atoms can form double or triple bonds.

Prediction of formulae

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