# NCERT Class 9 Science Solutions: Chapter 3 –Atoms and Molecules Part 3

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Question 2:

Q. Calculate the formula unit masses of given atomic masses of

A. Formula unit mass of

Formula unit mass of

Formula unit mass of

Question 1:

Q. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

A. One mole of carbon atoms weight 12 g (Given)

i.e., mass of 1 mole of carbon atoms

Then, mass of number of carbon atoms

Therefore, mass of 1 atom of carbon g

Question 2:

Q. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of ?

A. Atomic mass of

Then, gram atomic mass of

Now, 23 g of Na Contains number of atoms

Thus, 100 g of Na contains number of atoms

Again, atomic mass of

Then, gram atomic mass of

Now, 56 g of Fe contains number of atoms

Thus, 100 g of Fe contains number of atoms

Number of atoms

Therefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.

Question 1:

Q. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

A. Mass of boron

Mass of oxygen

Mass of sample

Thus, percentage of boron by weight in the compound

And, percentage of oxygen by weight in the compound

Question 2:

Q. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

A.

3 g of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide.

If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive.

In this case also, only 11 g of carbon dioxide will be formed.

The above answer is governed by the law of constant proportions.

Question 3:

What are polyatomic ions? Give example?

A. A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion, hydroxide ion, carbonate ion, sulphate ion.

Question 4:

Q. Write the chemical formulae of the following:

(a) Magnesium chloride

A. Magnesium chloride

(b) Calcium oxide

A. Calcium oxide

(c) Copper nitrate

A. Copper nitrate

(d) Aluminium chloride

A. Aluminium chloride

(e) Calcium carbonate

A. Calcium carbonate

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