# Question 75316

Jun 28, 2015

#### Answer:

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;

Some usefull examples:

http://socratic.org/questions/what-are-the-formulae-related-to-mole-calculations-answer-fast-plz-have-an-exam-

http://socratic.org/questions/how-many-moles-are-in-360-g-of-glucose-c6h12o6-how-would-i-be-able-to-find-the-a-1

http://socratic.org/questions/what-is-the-mass-in-grams-of-one-atom-copper-how-many-copper-atoms-are-in-1-gm-o

http://socratic.org/questions/how-many-moles-of-molecular-phosphorous-are-contained-in-92-91g-phosphorus-if-th

http://socratic.org/questions/how-many-iron-atoms-are-in-0-550-mol-of-fe2o3