Olympiad Test: Atoms and Molecules- 3 - Class 9 MCQ

We know that 1 mole of an atom = 6.023×10²³ atoms

1 mole of an atom = its atomic weight

Mass of one atom of an element = 2.65×10^–23 g

Mass of 6.023×10²³ atoms of an element Z = 2.65×10^–23 ×6.023×10²³ g =15.69 g

In CO₂, 12 g of carbon combines with 32 of oxygen.

12/32 = 3/8 = 3.8

Therefore, weight ratio of carbon to oxygen is 3:8. This is in agreement with the law of definite proportion.

• The first step in this problem is to change the % to grams.

  32.65%→32.65g of S

  65.3%→65.3g of O

  2.04%→2.04g of H

• Next divide all the given masses by their molar mass.

  32.65g of S/ 32gm-1 = 1.0203 moles of S

  65.3g of O/ 16gm-1 = 4.08 moles of O

  2.04g of H/ 1.008gm-1 = 2.024 moles of H

• Then, pick the smallest answer in moles from the previous step and divide all the answers by that. Remember that if you calculate a number that is x0.9 round to the nearest whole number

  1.0203 moles of S/ 1.0203 = 1

  4.08 moles of O/1.0203 = 3.998 ≈ 4

  2.024 moles of H/1.0203 = 1.984 ≈ 2

• Lastly, the coefficients calculated in the previous step will become the subscripts in the chemical formula.

  S = 1

  O = 4

  H = 2

  H₂SO₄

We know that :

6.022×10²³ molecules of Sulphur Dioxide = 1 mole

So, 12.044×10²² molecules of Sulphur Dioxide

= 1/6.022×10^{23 ×} 12.044×10²²

= 2/10

= 0.2 mole

Thus, 12.044×10²³ molecules of Sulphur Dioxide are 0.2 mole.

Given: 0.2moles of sodium.

To find: We have to find the number of atoms.

Solution:

To determine the number of atoms we have to follow the below steps-

Sodium is an alkali metal. It has the symbol Na.

It has the atomic number 11.

It has a mass of 23.

1 mole of sodium contains 6.023×10²³ number of atoms.

Thus 0.2 moles of sodium will contain

6.023×10²³ × 0.2 = 1.20×10²³ number of atoms.

Thus 0.2 moles of sodium contains 1.2×10²³ i.e., 13.054 × 10²² number of atoms.

We know that 1 mole of an atom = 6.023×10²³ atoms

1 mole of an atom = its atomic weight

Mass of one atom of an element = 2.65×10^–23 g

Mass of 6.023×10²³ atoms of an element Z = 2.65×10^–23 ×6.023×10²³ g =15.69 g