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SQA National 5 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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What is the empirical formula of glucose if it has a molecular formula of C₆H₁₂O₆?

SQA National 5 Chemistry Formulae What is the empirical formula of glucose if it has a molecular formula of C₆H₁₂O₆?
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2

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Do CO and COβ‚‚ have the same empirical formula?

SQA National 5 Chemistry Formulae Do CO and COβ‚‚ have the same empirical formula?
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3

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What is a common name for compound represented in the image?

SQA National 5 Chemistry Formulae What is a common name for compound represented in the image?
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4

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If you were to write an equation for a reaction with sand, what chemical formula would you use to represent it?

SQA National 5 Chemistry Formulae If you were to write an equation for a reaction with sand, what chemical formula would you use to represent it?
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5

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You may have noticed a trend of a decreasing number of hydrogens in compounds when going from left to right in the second period of the periodic table. Use this trend to predict a compound that fluorine would form. Give your answer as a chemical formula.

SQA National 5 Chemistry Formulae You may have noticed a trend of a decreasing number of hydrogens in compounds when going from left to right in the second period of the periodic table. Use this trend to predict a compound that fluorine would form. Give your answer as a chemical formula.
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6

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What is the name of ion CO₃²⁻?

SQA National 5 Chemistry Formulae What is the name of ion CO₃²⁻?
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7

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What is the chemical formula for a nitrate ion?

SQA National 5 Chemistry Formulae What is the chemical formula for a nitrate ion?
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8

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Water molecule may be written as HOH instead of Hβ‚‚O. Using this information, identify the two ions that a water molecule could be made of. Give your answer as thee names of the ions.

SQA National 5 Chemistry Formulae Water molecule may be written as HOH instead of Hβ‚‚O. Using this information, identify the two ions that a water molecule could be made of. Give your answer as thee names of the ions.
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9

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What is the formula of aluminium sulphide?

SQA National 5 Chemistry Formulae What is the formula of aluminium sulphide?
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10

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

SQA National 5 Chemistry Formulae What is the molecular formula of a covalent compound that boron and fluorine would form?
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