Chapter 3: Atoms and Molecules- Exercise- 40

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Chapter 3: Atoms and Molecules

Class: IX

Exercise number 40

Question 1

Write down the formula of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium suphide

(iv) magnesium hydroxide

Answer 1

(i) Sodium oxide → Na₂O

(ii) Aluminium chloride → AlCl₃

(iii) Sodium suphide → Na₂S

(iv) Magnesium hydroxide → Mg(OH)₂

Question 2

Write down the names of compounds represented by the following formula:

(i) Al₂(SO₄)₃

(ii) CaCl₂

(iii) K₂SO₄

(iv) KNO₃

(v) CaCO₃

Answer 2

(i) Al(SO₄)₃→ Aluminium sulphate

(ii) CaCl₂→ Calcium chloride

(iii) K₂SO₄→ Potassium sulphate

(iv) CaCO₃→ Calcium carbonate

Question 3

What is meant by the term chemical formula?

Answer 3

The chemical formula of a compound is the symbolic representation of the composition of a compound. We can know the number and kinds of atoms of different elements in the compound from a chemical formula.

 For example, from the chemical formula CO₂ ( carbon dioxide), we can see that, there are  one carbon atom and two oxygen atoms  in one molecule of carbon dioxide.

Question 4

How many atoms are present in a

(i) H2S molecule and

(ii) PO₄^3- ion?

Answer 4

(i) In an H₂S molecule, three atoms are present; two of hydrogen and one of sulphur.

(ii) In a PO₄ ^3- ion, five atoms are present; one of phosphorus and four of oxygen.