# Question 1e5b3

Jul 1, 2017

#### Answer:

$2.4 \cdot {10}^{24}$

#### Explanation:

The first thing that you need to do here is to convert the mass of water to moles by using the compound's molar mass.

36 color(red)(cancel(color(black)("g"))) * ("1 mole H"_2"O")/(18.015color(red)(cancel(color(black)("g")))) = "1.998 moles H"_2"O"

Now, you know that very mole of water contains

• two moles of hydrogen, $2 \times \text{H}$
• one mole of oxygen, $1 \times \text{O}$

This means that your sample contains

1.998 color(red)(cancel(color(black)("moles H"_2"O"))) * "2 moles O"/(1color(red)(cancel(color(black)("mole H"_2"O")))) = "3.996 moles O"#

Finally, to find the number of atoms of oxygen present in the sample, use the fact that $1$ mole of elemental oxygen must contain $6.022 \cdot {10}^{23}$ atoms of oxygen $\to$ this is the definition of a mole given by Avogadro's number.

You will end up with

$3.996 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{moles O"))) * (6.022 * 10^(23)color(white)(.)"atoms O")/(1color(red)(cancel(color(black)("mole O")))) = color(darkgreen)(ul(color(black)(2.4 * 10^(24)color(white)(.)"atoms O}}}}$

The answer is rounded to two sig figs.