AP Chemistry - Using Moles

Consider the following four samples:

$\small 50g$ of potassium

$\small 50g$ of lithium

$\small 50g$ of magnesium

$\small 50g$ of chlorine gas

Which of the given samples contains the most atoms?

Lithium

Potassium

Chlorine

They all have the same number of atoms

Explanation

It is important to note that the mass of a sample does not tell you the amount of atoms in the sample. The number of atoms in a sample is dependent on the number moles in a sample, given by Avogadro's number. Here is the number of moles for each sample:

$50 g\ Li*\frac{1 mol\ Li}{6.94 g\ Li} = 7.2 mol\ Li$

$50 g\ Mg*\frac{1 mol\ Mg}{24.31 g\ Mg} = 2.1 mol\ Mg$

$50 g\ K*\frac{1 mol\ K}{39.10 g\ K} = 1.3 mol\ K$

$50 g\ Cl_2*\frac{1 mol\ Cl_2}{70.90 g\ Cl_2} = 0.71 mol\ Cl_2$

Remember that chlorine is a diatomic mass, so each molecules contains two atoms. This doubles the molar mass for the conversion.

The sample with the greatest number of moles will also contain the most atoms. In this case, the sample of lithium results in the largest number of moles and, thus, the greatest number of atoms.

Convert the following amount from grams (g) to moles (m)

How many moles is of ?

Explanation

Use the periodic table to calculate the molecular weight of sodium hydroxide.

$MW_{NaOH}= 39.99\frac{g}{mol}$

Next, use dimensional analysis to find the number of moles.

$357g:NaOH\cdot\frac{1mol}{39.99g}=8.93mol$

$2KBr\ +\ Cl_2\ \rightarrow\ 2KCl\ +\ Br_2$

Consider the reaction above. If you start with of potassium bromide, how many moles of bromine are produced? How many molecules is this equal to?

$2 mol=1.204\cdot 10^{24} molecules$

$8 mol=4.816\cdot10^{24}molecules$

Explanation

In the chemical equation, the ratio of potassium bromide to bromine is 2:1, so for every 2 moles of , 1 mole of is produced. Therefore, if we start with 4 moles of , we get 2 moles of . The number of molecules is equal to the number of moles times Avogadro's Number. Since we've determined the number of moles to be 2, the number of molecules is: