Please explain about the way to calculate the number of atoms of a certain element in a given mass of a compound?

It's all based on the definition:

`"1 mol of atoms" = 6.022 × 10^23 color(white)(l)"atoms"`

Let's say the question is,

How many oxygen atoms are in 12.3 g of `"KClO"_3`?

Step 1. Calculate the molar mass of `"KClO"_3`

`M_text(r) = "1K + 1Cl + 3O" = "1×39.10 + 1×35.45 + 3×16.00" = 122.55`

So, 1 mol of `"KClO"_3` has a mass of 122.55 g.

Step 2. Calculate the moles of `"KClO"_3`

`"Moles of KClO"_3 = 12.3 color(red)(cancel(color(black)("g KClO"_3))) × (1 "mol KClO"_3)/(122.55 color(red)(cancel(color(black)("g KClO"_3)))) = "0.1004 mol KClO"_3`

Step 3. Calculate the moles of `"O"` atoms

We see that one formula unit of `"KClO"_3` contains three "O" atoms. So,

`bb("1 mol KClO"_3 = "3 mol O atoms")`

∴ `"Moles of O atoms" = 0.1004 color(red)(cancel(color(black)("mol KClO"_3))) × "3 mol O atoms"/(1 color(red)(cancel(color(black)("mol KClO"_3)))) = "0.3011 mol O atoms"`

Step 4. Calculate the number of `"O"` atoms

`"No. of O atoms" = 0.3011 color(red)(cancel(color(black)("mol O atoms"))) × (6.022 × 10^23 color(white)(l)"O atoms")/(1 color(red)(cancel(color(black)("mol O atoms"))))`

`= 1.81 × 10^23color(white)(l)"O atoms"`

Thus, `"12.3 g of KClO"_3` contains `1.81 × 10^23 color(white)(l)"O atoms"`.

Here is a video on converting moles of a compound to atoms.