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

May 5, 2018

Here's how I do it.

#### Explanation:

It's all based on the definition:

$\text{1 mol of atoms" = 6.022 × 10^23 color(white)(l)"atoms}$

Let's say the question is,

How many oxygen atoms are in 12.3 g of ${\text{KClO}}_{3}$?

Step 1. Calculate the molar mass of ${\text{KClO}}_{3}$

${M}_{\textrm{r}} = \text{1K + 1Cl + 3O" = "1×39.10 + 1×35.45 + 3×16.00} = 122.55$

So, 1 mol of ${\text{KClO}}_{3}$ has a mass of 122.55 g.

Step 2. Calculate the moles of ${\text{KClO}}_{3}$

${\text{Moles of KClO"_3 = 12.3 color(red)(cancel(color(black)("g KClO"_3))) × (1 "mol KClO"_3)/(122.55 color(red)(cancel(color(black)("g KClO"_3)))) = "0.1004 mol KClO}}_{3}$

Step 3. Calculate the moles of $\text{O}$ atoms

We see that one formula unit of ${\text{KClO}}_{3}$ contains three "O" atoms. So,

$\boldsymbol{\text{1 mol KClO"_3 = "3 mol O atoms}}$

$\text{Moles of O atoms" = 0.1004 color(red)(cancel(color(black)("mol KClO"_3))) × "3 mol O atoms"/(1 color(red)(cancel(color(black)("mol KClO"_3)))) = "0.3011 mol O atoms}$

Step 4. Calculate the number of $\text{O}$ 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, ${\text{12.3 g of KClO}}_{3}$ contains 1.81 × 10^23 color(white)(l)"O atoms".

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