Question

How many number of potassium atoms are present in 7.8 g of potassium element?
1. 12 × 10²³
2. 12 × 1024
3. 1.2 × 1023
4. 1.2 × 1022

Answer 1

Correct Answer - Option 3 : 1.2 × 1023

Concept:
Avogadro's number: The number of units present in one mole of a given substance is called the Avogadro's number.

• Avogadro's number (NA) is equal to 6.022140857 × 1023.
• Avogadro's number is the no of atoms, elements or molecules present in 1 mole.
• Mass of 1 mole of substance = Atomic mass of individual

Calculation:

• Atomic mass of potassium atoms = 39
• Mass of 1 mole of potassium atoms = atomic mass of potassium atom

Mass of 1 mole of potassium = 39 g

39g potassium contain atom = 6.022 × 10²³

1g potassium will contain atom = 6.022 × 1023 / 39

Thus 7.8 g potassium will contain atom = (6.022 × 1023  / 39) × 7.8

⇒ 1.2 × 1023