NCERT · Class 9 · Science

Chapter 3: Atoms and Molecules

Practice each question, then open the answer or worked solution.

Exercise-3.5.3

Textbook page 42 · 2 questions

Q1Written answer

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

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Worked solution

Given: 1 mole of carbon weighs 12g

1 mole of carbon atoms = 6.022 x 10²³

The molecular mass of carbon atoms = 12g = an atom of carbon mass

Hence, mass of 1 carbon atom = 12 / 6.022 x 10²³ = 1.99 x 10^-23g

Q2Written answer

Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given the atomic mass of Na = 23u, Fe = 56 u)?

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Worked solution

(a) In 100 grams of Na:

m = 100g, Molar mass of Na atom = 23g, N₀ = 6.022 x 10²³, N = ?

N = (Given mass x N₀)/Molar mass

N = (100 x 6.022 x 10²³)/ 23

N = 26.18 x 10²³ atoms

(b) In 100 grams of Fe:

m = 100 g, Molar mass of Fe atom = 56 g, N₀ = 6.022 x 10²³, N = ?

N = (Given mass x N₀)/ Molar mass

N = (100 x 6.022 x 10²³)/ 56

N = 10.75 x 10²³ atoms

Therefore, the number of atoms is more in 100 g of Na than in 100 g of Fe.