■ Chemical Formulae
► | To represent a chemical compound. | |
► | The elements present in the compound which are denoted by their symbols. | |
► | The relative numbers of each element present in the compound which are indicated by subscripts written after the symbols. |
► | Several covalent compounds and their chemical formulae. | ||||||||||||||||||||||
► | Number of atoms of each element making up the compound. | ||||||||||||||||||||||
Example of electron configuration and valence electrons.
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(a) | Empirical formula. | |
(b) | Molecular formula. |
Empirical formula and Molecular formula
■ Empirical formula:
► | Definition: Empirical formula: Shows the simplest ratio of the atoms to the elements that combine to form the compound. |
► | Definition: Molecular formula: Shows the actual number of atoms of the elements that combine to form the compound. | |||||
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The molecular formula is the multiple of the empirical formula. , where n is an integer. |
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Example :
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► | Step 1: Find the mass or percentage of elements in compound. | |
► | Step 2: Divide mass or percentage by relative atomic mass(RAM) to find number of moles. | |
► | Step 3: Find smallest ratio by dividing the number obtained in Step 2 by the smallest number of all. | |
► | Step 4: Write empirical formula |
Laboratory Activity 3.5.1: Determining the empirical formula of copper oxide | |
Laboratory Activity 3.5.2: Determining the empirical formula of magnesium oxide |
✍ Worked-example 3.5(a)
In an experiment, a metal X combines with 4.32g oxygen to form 13.66g of a metallic oxide of X. Find the
empirical formula of the metallic oxide.[RAM: X = 52, O = 16] Solution
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✍ Worked-example 3.5(b)
A compound
contains 40% carbon and 53.3% oxygen. If the relative molecular mass of the compound is 180, find its (a) empirical formula (b) molecular formula [RAM: H, 1; C, 12; O = 16] Solution (a) Empirical formula
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(b) Molecular formula Assume that the molecular formula of compound is The relative molecular mass of X is 180. Thus, Thus the molecular formula of compound is or |
■ Method of writing ionic formulae
► | Can be written if the charge of the cation(positively charged ion) and the anion(negatively charged ion) forming the ionic compound are known. | ||||||||||||||||||||||
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(a) | Write the formulae of the ions involved in forming the compound and their charges. | |
(b) | Then balances the positive and negative charges. | |
(c) | Finally write the chemical formula of the ionic compound without the charges. |
■ Naming of chemical compound
► | Based on the recommendation of the International Union of Pure and Applied Chemistry(IUPAC) . |
► | The name of the cation comes first, followed by the name of the anion.
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► | Roman numerals are used in naming of the metals than form more than one type of ions.
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► | In naming of the simple molecular compound, the name of the first element comes first, followed by the second element
with an 'ide'.:
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► | Greek prefix are used to show the number of atoms of each element in a compound. |
⇲ For exercise(objective and subjective), download for free on Android OS. | ||
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