# Question 75316

Jun 28, 2015

Depends on the quantity.

#### Explanation:

Assuming that you're dealing with a mass of a substance, the first thing you need to do is determine how many moles are present in that mass.

Once you do that, you use the fact that 1 mole of any substance contains exactly $6.022 \cdot {10}^{23}$ atoms or molecules of that substance - this is known as Avogadro's number.

So, let's say that you want to determine how many molecules are present in 100. g of water.

Use water's molar mass, which tells you what the mass of 1 mole of water is, to determine how many moles you have in 100. g

100. cancel("g") * "1 mole water"/(18.02cancel("moles")) = "5.55 moles"

Now use Avogadro's number to determine how many molecules you have in 5.55 moles

5.55cancel("moles") * (6.022 * 10^(23)"molecules")/(1cancel("mole")) = 3.34 * 10^(24)"molecules"#

So remember, when having to determine how many molecules or atoms a certain mass contains, always

• use molar mass to determine the number of moles;
• use Avogadro's number to determine the number of molecules/atoms;

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