Question

Calculate the number of atoms $C,H$ and $O$ in $72.5g$ of iso-propanol ( $CH,OH$ ) having molar mass $60g$ mol ?

Answer 1

To solve the given question, we should have information regarding mole concept, atomicity and avagadro’s number. Mole is the amount of substance which contains present in $12gm$ of $C-12$ isotope of carbon.
Atomicity is the total number of elements present in a molecule or composed of a molecule. Avagadro’s number is the number of ions, atoms or molecules present in a mole of a substance. It has a constant value of $6.022 \times 10^{22}$ .
The formula for number of moles is :
Total-mass $= \dfrac{\text{Given mass}}{\text{Molar mass}}$
The other formula for the same is the number of moles and the number of molecules by Avagadro’s number denoted by $N_A$ .

Complete answer:
Step-1 :
We have given $72.5g$ of iso-propanol ($C_3H_7OH$) and molar mass of iso-propanol is given to be $60g$ mol. So, using the given formula, moles can be calculated as :
$moles = \dfrac{\text{Given mass}}{\text{Molar mass}}$
$moles = \dfrac{72.5}{60} = 1.2$
So, we have $1.2$ moles of iso-propanol.

Step-2 :
We also have another formula for number of moles :
$moles = \dfrac{\text{Total molecules}}{N_A}$
To calculate the number of atoms, multiplicity the number of molecules by atomicity.
For $C$ atoms we have, number of atoms :
$3 \times N_A \times 1.2 = 2.128 \times 10^{ 24 }$
For $H$ atoms we have, number of atoms :
$8 \times N_A \times 1.2 = 5.82 \times 10^{ 24 }$
For $O$ atoms we have, number of atoms :
$1 \times N_A \times 1.2 = 7.2 \times 10^{ 23 }$

Note:
We should know that a molecule is made up of a group of atoms considered as elements or compounds. To calculate the number of atoms, the number of molecules is multiplied by atomicity, i.e. atoms present in a molecule.