# Question 513f2

Sep 30, 2015

The answer is (a) $6.02 \cdot {10}^{23}$

#### Explanation:

So, you know that your sample of glass contains $\text{9 g}$ of water.

Use water's molar mass to find how many moles of water you get in your glass sample.

9color(red)(cancel(color(black)("g"))) * "1 mole water"/(18.02color(red)(cancel(color(black)("g")))) = "0.4994 moles water"

A water molecule, $\text{H"_2"O}$, contains one oxygen atom and twp hydrogen atoms. This means that every mole of water, you have

• one mole of oxygen
• two moles of hydrogen

This means that the number of moles of hydrogen you have in that many moles of water will be

0.4994color(red)(cancel(color(black)("moles water"))) * "2 moles H"/(1color(red)(cancel(color(black)("mole water")))) ~~ "1.00 moles H"

Now, one mole of any element contains exactly $6.022 \cdot {10}^{23}$ atoms of that element - this is known as Avogadro's number.

In your case, one mole of hydrogen will contain

1.00color(red)(cancel(color(black)("mole H"))) * (6.022 * 10^(23)"atoms H")/(1color(red)(cancel(color(black)("mole H")))) = color(green)(6.02 * 10^(23)"atoms of H")#

SIDE NOTE The exact value will actually be $6 \cdot {10}^{23}$ atoms of hydrogen, since you only gave one sig fig for the mass of water.