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Question

Explain how to calculate the number of atoms of a certain element in a given mass of a compound.


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Solution

The number of atoms:

  • To calculate the number of atoms first step is to calculate the number of moles.
  • If the mass of an element/compound is given then divide the given mass by the molar mass of the element/compound to find the number of moles.
  • In 1 mole of a substance, the number of atoms is NAor 6.023×1023 atoms.
  • Multiply the given moles with NA to find the total number of atoms.

Example 1: To calculate the number of atoms in 46g of Na

Step-1: Calculate the number of moles

  • Number of moles = Given massMolar mass=4623=2 mol

Step 2: Calculate the number of atoms

  • The number of atoms= number of moles× NA=2×6.023×1023= 12.046×1023=1.2046×1023atoms.

Example 2: To calculate the number of atoms of C in 22g of CO2

Step-1: Calculate the number of moles

  • Number of moles = Given massMolar mass=2244=0.5 mol

Step 2: Calculate the number of atoms

  • The number of atoms= number of moles× NA
  • Since CO2 is triatomic. i.e 1 molecule of CO2 contain 1 atom of C and 2 atom of O
  • The total number of atoms =0.5×3×6.023×1023 = 9.0345× 1023 atoms.
  • The number of atoms of C = 0.5×1×6.023×1023 =3.0115× 1023 atoms.

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