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Formulae for Edexcel GCSE Chemistry

Formulae

This page covers the following topics:

1. Empirical and molecular formulae
2. Ions
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 โ€œ1โ€ (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.

Empirical and molecular formulae

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.

OHโป is hydroxide,
COโ‚ƒยฒโป - carbonate,
NOโ‚ƒโป - nitrate,
POโ‚„ยณโป - phosphate,
SOโ‚„ยฒโป - sulphate,
SOโ‚ƒยฒโป - sulphite,
NHโ‚„โบ - 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.

Ions

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:

Hโ‚‚O is water,
NHโ‚ƒ - ammonia,
CHโ‚„ - methane,
COโ‚‚ - carbon dioxide,
SiOโ‚‚ - 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; โ€œideโ€ for single-atom ions like sulphide, โ€œateโ€/โ€œiteโ€ 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.

Predicting formulae

1

What is a difference between SOโ‚ƒ and SOโ‚ƒยฒโป?

SOโ‚ƒ is a neutral molecule, while SOโ‚ƒยฒโป is a negatively charged ion that had to gain several electrons to acquire the charge.

SOโ‚ƒ is a molecule, SOโ‚ƒยฒโป is an ion.

What is a difference between SOโ‚ƒ and SOโ‚ƒยฒโป?

2

What is the molecular formula of a covalent compound that boron and fluorine would form?

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.

Boron has 3 unpaired electrons in its outer shell; thus, its valency is 3. Fluorine has 1 unpaired electron in its outer shell; therefore, its valency is 1. Cross-multiplying the valencies of boron and fluorine gives BFโ‚ƒ. Since the subscripts have a common factor of 1, they cannot be simplified.

BFโ‚ƒ

What is the molecular formula of a covalent compound that boron and fluorine would form?

3

Provide the name of the ion presented in the image.

Carbonate ion has a formula of COโ‚ƒยฒโป.

carbonate

Provide the name of the ion presented in the image.

4

Do NOโ‚‚ and Nโ‚‚Oโ‚„ have the same empirical formula?

To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden โ€œ1โ€ (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.

For NOโ‚‚ the highest common factor of 1 and 2 is 1. Dividing each of the subscripts by 1 gives 1 and 2. The โ€œ1โ€ is excluded from the empirical formula to give NOโ‚‚.

For Nโ‚‚Oโ‚„ the highest common factor of 2 and 4 is 2. Dividing each of the subscripts by 2 gives 1 and 2. The โ€œ1โ€ is excluded from the empirical formula to give NOโ‚‚.

yes

Do NOโ‚‚ and Nโ‚‚Oโ‚„ have the same empirical formula?

5

What is the empirical formula of glucose if it has a molecular formula of Cโ‚†Hโ‚โ‚‚Oโ‚†?

To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden โ€œ1โ€ (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.

The highest common factor of 6, 12 and 6 is 6. Dividing each of the subscripts by 6 gives 1, 2 and 1. The โ€œ1โ€s are excluded from the empirical formula to give CHโ‚‚O.

CHโ‚‚O

What is the empirical formula of glucose if it has a molecular formula of Cโ‚†Hโ‚โ‚‚Oโ‚†?

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