2. Write down the chemical formula of the following compounds using FOUR STEP METHOD. (a) Potassium oxide (b) Sodium hydrogen carbonate (c) Calcium sulphate (d) Ammonium carbonate
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Answer:
(a)The formula for potassium oxide is K2O. The potassium ion has a charge of 1+ and the oxygen ion has a charge of 2-. For an electrically neutral ionic compound the charges must balance. Therefore, there must be two k+ ions for every oxygen 2- ion.
(b) 
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General Chemistry

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Write the chemical formula of sodium carbonate.
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Hint:
The symbolic representation of the composition of a compound is known as its chemical formula. It tells us about the number of different atoms of various elements that are present in one molecule of a compound. If you know the symbol and valencies of the elements it will be easy for you to write a chemical formula.
Complete answer:
Step 1
The abbreviated name of an element or to write the name of an element in its short form is known as symbol. For example the symbol of nitrogen in N, oxygen is O, sodium is Na, carbon is C, etc.
Step 2
The combining property of an element i.e. how an element combines with other elements is known as its valency. We measure valency in terms of hydrogen atoms or oxygen atoms. We calculate valency by counting the number of times hydrogen or oxygen which can combine with that element.
Hydrogen has the least combining capacity, its valency is taken as standard and considered to be one.
Step 3
According to the modern concept, the number of electrons that an atom can lose, gain, or share during a chemical reaction is called its valency. Many elements have variable valencies i.e. they have more than one valency.
Step 4
An ion is a positively or negatively charged atom/ or group of atoms that are formed by loss or gain of electrons by an atom or group of atoms. Positively charged ions formed by losing electrons are known as cations and negatively charged ions formed by gaining electrons are known as anions.
Step 5
The steps for writing a formula are:
Write the symbols of the elements or ions that form the compound.
You have to write the valency of each element or the charge of each ion below its symbol.
Finally, you have to interchange the valencies or charges of the combining atoms or ions and write them as subscripts.
If you find that the valencies or charges are divisible by a common factor, then you have to divide and simplify the formula.
Hence, applying the above rules the chemical formula for sodium carbonate is:
Symbol
Na+1Na+1
(sodium ion)
(CO
(c)Calcium = Ca, it has valency of +2
Sulphate = SO4, valency combined is -2
So calcium sulphate = Ca(+2) SO4(2-)
+2 nd -2 cancels
So formula for calcium sulfate is CaSO4
Explanation:
Formula of Potassium Oxide is
K^2O
Valency of Potassium =1 (as its atomic no. is 19 that means its valence shell contains 1 e & it is easy to lossing 1e than gaining 7e).
Valency of Oxygen =2(similarly , Oxygen has atomic no. 8 and it gains 2e).
Now,
K O
/ \
1 2
=K^2O