Writing Of Formulae Of Molecular Compounds
If we know the valencies of elements,then we can work out the formulae of their compounds by balancing the valencies of the different atoms which occur in the ompound. For example, if a compound is made up of hydrogen and sulphur elements, then we should adjust the number of hydrogen atoms and sulphur atoms in such a way that the total valencies of hydrogen atoms become equal to the total valencies of sulphur atoms.This will give us the correct formula of the compound.Let us work out the formula of hydrogen sulphide to make this point more clear.
Hydrogen sulphide compound is made up of two elements : hydrogen and sulphur.Now,the valency of hydrogen is 1 and that of sulphur is 2.That is,
Valency of one H atom = 1
And Valency of one S atom = 2
Now, to balance the valency of 2 of a sulphur atom,we will have to take 2 atoms of hydrogen, because :
Valency of two H atoms =1x2
= 2
This means that 2 atoms of H will combine with 1 atom of S to form the compound .Thus, the formula of hydrogen sulphide is .
We will now describe the 'crossing-over of valencies' method of working out the formulae of molecular compounds. In this method :
1.We first write the symbols of the elements which form the compound.
2.Below the symbol of each element, we write down its valency.
3.Finally, we cross-over the valencies of the combining atoms.That is,with first atom we write the valency of second atom (as a subscript); and with the second atom we write the valency of first atom (as subscript). This will give us the required formula.
This method of writing the formulae will become more clear from the following example. Let us work out the formula of hydrogen sulphide by this method.
1.Hydrogen sulphide compound is made up of hydrogen and sulphur elements.So,first we write down the symbols of hydrogen and sulphur.
2.The valency of hydrogen is 1 and the valency of sulphur is 2. So, below the symbol H we write 1 and below the symbol S we write 2 :
3. We now cross-over the valencies of H and S atoms. With H atom we write the valency of S (which is 2) as a subscript so that it becomes . With S atom we write the valency of H (which is 1) as a subscript so that it becomes . Now, joining together and the formula of hydrogen sulphide becomes or (This is because we do not write the subscript 1 with an atom in a formula).Let us solve some problems now.
Sample Problem 1. Work out the formula of carbon dioxide.
Solution. We can work out the formula of carbon dioxide as follows :
1.Carbon dioxide is a compound made up of carbon and oxygen elements. So, first we write down the symbols of carbon (C) and oxygen (O).
2.The valency of carbon is 4 and the valency of oxygen is 2. So, below the symbol C we write 4 and below the symbol O we write 2.
3. We now cross-over the valencies of C and O atoms. The formula becomes . This formula contains 2 carbon atoms and 4 oxygen atoms having a common factor of 2. So, dividing this formula by the common factor of 2, we get the simplest formula .Thus, the formula of carbon dioxide is .
Dear student, we have worked out the formula of carbon dioxide by giving so many details just to make you understand the various steps clearly. After a little more practice, these is no need to write so many steps.
Sample Problem 2. Work out the formula of hydrogen chloride.
Solution. Hydrogen chloride is made up of hydrogen and chlorine elements.The symbol of hydrogen is H and that of chlorine is Cl. The valency of hydrogen is 1 and the valency of chlorine is also 1. The formula of hydrogen chloride can be worked out as follows :
Sample Problem 3. The valency of carbon is 4 and that of chlorine is 1.What will be the formula of carbon tetrachloride ?
Solution. We can work out the formula of carbon tetrachloride as follows :
Thus, the formula of carbon tetrachloride is .
Sample Problem 4.The valency of an element X is 1 and that of oxygen is 2. What will be the formula of the compound formed by the combination of element X with oxygen ?
Solution. The formula of compound of element X with oxygen can be found as follows :
Thus, the formula of compound of element X with oxygen is .
(Please note that the element X of valency 1 is like hydrogen, H.So,the compound may be like water, )
Sample Problem 5. An element E has a valency of 4.
(i) What will be the formula of its chloride ?
(ii) What will be the formula of its sulphide ?
Solution. In order to solve this problem,we should know the valencies of chlorine and sulphur elements (because chlorine forms the chloride and sulphur forms the sulphide). Knowing that the valency of chlorine is 1 and that of sulphur is 2, the above problem can be solved as follows :
(ii) Formula of sulphide of element E
Thus,the formula of chloride of element E is .
(ii) Formula of sulphide of element E Thus, the formula of sulphide of element E is .
(It should be noted that the element E having a valency of 4 is like carbon,C.So,the choride is like carbon tetrachloride,,and the sulphide, ES2, is like carbon disulphide,).
Sample Problem 6.An element X shows two valencies of 3 and 5. Work out the formulae of two oxides of this element.
Solution. In order to find out the formulae of the oxides of element X, we should know the valency of oxygen element (because an element combines with oxygen element to form the oxide). Knowing that the valency of oxygen is 2, we can write down the formulae of the two oxides of element X as follows :
(a) Formula of oxide of element X when its valency is 3Thus, the formula of oxide of element X when its valency is 3 is .
(b) Formula of oxide of element X when its valency is 5
Thus, the formula of oxide of element X when its valency is 5 is .
(Please note that the element X of the above problem having valencies of 3 and 5 is like phosphorus, P,so that the oxide is like phosphorus trioxide,,and the oxide is like phosphorus pentoxide,).
Sample Problem 7. An element Z forms an oxide .
(a) What is the valency of element Z ?
(b) What will be the formula of fluoride of Z ?
Solution,(a) The compound has three atoms of oxygen. Now :
Valency of one O atom = 2
So,Valency of three O atoms = 2x3
= 6
Now, in the compound the valency of Z should be equal to the total valencies of O. Since the total valency of oxygen atoms in this compound is 6, therefore, the valency of element Z should also be 6.(b) Formula of the fluoride of element Z
When element Z combines with fluorine, then its fluoride is formed. Now, valency of element Z is 6 and the valency of element fluorine, F, is 1.So, the formula of fluoride of element Z can be worked out as follows :
Thus, the formula of fluoride of element Z is .
(Please note that the element Z of this problem is like sulphur element S, so that the oxide is like sulphur trioxide,,and fluoride is like sulphur hexafluoride,).
Notes
Atoms and Molecules - Notes
2. Monovalent Cations (Cations Having a Valency of 1+)
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3. Divalent Cations (Cations Having a Valency of 2+)
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4. Trivalent Cations (Cations Having a Valency of 3+)
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6. an example, let us give the significance of symbol C
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7. Formulae of Some Molecular Compounds
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9. Explanation of the Law of Conservation of Mass
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10. Molecular Formulae of Some Common Elements
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11. 2. A Negatively Charged Ion is Known as Anion
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13. As an example, let us give the significance of the formula H
2O
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14. Simple Ions and Compound Ions (Polyatomic Ions)
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21. Gram Atomic Mass And Gram Molecular Mass
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28. 1. A Positively Charged Ion is Known as Cation
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34. Divalent Anions (Anions Having a Valency of 2-)
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35. Molecular Masses of Some Common Elements
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37. Explanation of the Law of Constant Proportions
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39. Significance of the Formula of a Substance
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42. Symbols Derived from Latin Names of the Elements
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45. Valencies of Some Common Metal Elements Valencies of Some Common Non-Metal Elements
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46. Atomic Masses of Some Common Elements
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47. Writing Of Formulae Of Molecular Compounds
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50. Writing Of Formulae Of Ionic Compounds
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