1.1 What is Chemistry?

http://www.ck12.org Chapter 10. The Mole

N: 1 x 14.0 g/mol = 14.0 g/mol

O: 3 x 16.0 g/mol = 48.0 g/mol

AgNO 3 - 169.9 g/mol

Because 169.9 grams of AgNO 3 is equivalent to one mole of AgNO 3 , we can use that as a conversion factor. We
start with the amount that we need (2.0 moles), and convert to the desired units (grams of AgNO 3 ):

2 .0 moles AgNO 3 ×^169 1 mole AgNO.9 grams AgNO 3 3 = 339 .8 grams AgNO 3

The units of moles cancel, and we are left with grams.

Example 10.7

In another reaction, we determine that we need 0.45 moles of NaBr. We check our chemical supplies and find we
have 37.2 grams NaBr on hand. Do we have enough for this reaction?

Na: 1 x 23.0 g/mol = 23.0 g/mol

Br: 1 x 79.9 g/mol = 79.9 g/mol

NaBr - 102.9 g/mol

0 .45 moles NaBr×^102 1 mole NaBr.9 grams NaBr= 46 .3 grams NaBr

We need 0.45 moles, which our calculation tells us is equivalent to 46.3 grams. We only have 37.2 grams available,
so we do not have enough for this experiment.

Example 10.8

We run an experiment that gives us 65.4 grams of Rb 2 O as a product. How many moles did we obtain?

First, we need to calculate the molar mass of the compound:

Rb: 2 x 85.5 g/mol = 171.0 g/mol

O: 1 x 16.0 g/mol = 16.0 g/mol

Rb 2 O - 187.0 g/mol

65 .4 g Rb 2 O× 187 1 mol Rb.0 g Rb^2 O 2 O= 0 .350 mol Rb 2 O

We obtained 0.350 moles of Rb 2 O. Note that the conversion factor of molar mass (187.0 grams Rb 2 O = 1 mole
Rb 2 O) was written "upside-down," with the grams on the bottom and the moles on top. This is so that the unwanted
units (grams) cancel, leaving only the desired units (moles) in our final answer. Whenever we have this type of
conversion factor, the choice of which quantity to put in the numerator and denominator depends on the units that
we wish to cancel.

Volume and Moles

Avogadro’s Hypothesis

Avogadro proposed that equal volumes of gases at the same temperature and pressure contain the same number of
particles, and therefore the same combining ratios. This means that at a given temperature and pressure, one mole
of any gas will take up the same volume, regardless of its identity. This is very different than the case for solids and
liquids. For example, a mole of water takes up significantly more space than a mole of gold, which is quite dense.