AQA A-level Chemistry Formulae
This page covers the following topics:
1. Empirical and molecular formulae
3. Predicting formulae
The empirical formula represents the simplest whole number ratio of elements in a compound. The molecular formula denotes all the atoms of each element present in a compound. To get a molecular formulae from diagrams, count the number of atoms of each kind.
To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden “1” (when there is just a single atom of some element in a compound) and divide all of the subscripts in the formula by this greatest common factor.
To determine the empirical formula from a percentage composition, take a 100 g of a material, in which case mass in grams of each element will be the same as the percentage. Find the number of moles for each element by dividing by the mass number of an atom, and divide each by the smallest number of moles you got. You may multiply the ratio you get from the divisions by additional numbers to get integers.
To convert an empirical formula to a molecular formula, find how many times the relative formula mass of the compound is greater than the relative formula mass of the empirical formula. Then multiply the numbers of atoms in the empirical formula by that number to get the molecular formula.
An ion is an atom or a molecule that has lost or gained electrons and has either a positive or negative charge. 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.
Since the group an element is in indicates how many electrons are in the outer shell, it can be used to determine the ionic charge. Metals usually lose electrons to form positive ions with the charge of their group number and a plus (+). Nonmetals are more likely to gain electrons to form negative ions with their charge being their group number minus 8.
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:
H₂O is water,
NH₃ - ammonia,
CH₄ - methane,
CO₂ - carbon dioxide,
SiO₂ - sand.
The formulae of ionic compounds can be predicted by cross-multiplying the charges of the anion and cation and dividing them by a common factor. The name for ionic compounds is derived by combining the names of the two ions, using a suitable suffix for the anion; “ide” for single-atom ions like sulphide, “ate”/“ite” for group ions like sulphate.
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 such compound make a fixed number of covalent bonds with some of them being a part of a double or a triple bond.
What is a difference between SO₃ and SO₃²⁻?
What is the molecular formula of a covalent compound that boron and fluorine would form?
Provide the name of the ion presented in the image.
Do NO₂ and N₂O₄ have the same empirical formula?
What is the empirical formula of glucose if it has a molecular formula of C₆H₁₂O₆?
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