1. Individual count  / Avogadro’s number  Mole count n = N / NA
QUESTION:
How many moles of O2 molecules are in a sample containing 3.0x1022 O2 molecules?
A mol, B. 1.06x10-21 mol, C mol
Definition: 1 mol = x1023
Mole count
1 mol =
Individual count
x 1023
How many atoms of carbon are in a 2.50 mol sample of carbon?
A x1024, B. 30.0, C. 4.15x10-22
PAUSE CLICK
One mole is defined as a group count equivalent to Avogadro’s number, which to six significant digits is x1023
Note that the official abbreviation for mole is em oh ell.... no E.
HIGHLIGHT mol
It should be obvious that a mole count and individual counts are directly proportional quantities...
If you have zero molecules, you also have zero moles of molecules.
If you have twice as many molecules, then the moles of molecules also doubles.
When two quantities are directly proportional, their ratio is constant.
In other words, we can say that the ratio of
the mole count
CLICK
to the individual count
CLICK CLICK
is equal to
the ratio of 1 mole
to Avogadro’s number.
We can rearrange this equation to solve for the mole count. Essentially, what we would end up doing is
multiply the given individual count, 3.0x1022
by the ratio of 1 mole to Avogadro’s number
CLICK CLICK CLICK
The ratio serves as a conversion factor.
Note that we rounded our value for Avogadro’s number to three significant digits.
HIGHLIGHT 6.02
There’s no need to use more because our less precise term has only 2 significant digits.
CALLOUT “2 sig.figs.” pointing to 3.0x1022
The answer we get should be rounded off to two significant figures... and it’s equal to 0.50 moles
TO summarize,
CLICK if we want to convert an individual count to a mole count,
we simply divide the individual count by by Avogadro’s number.
CLICK If capital N is the individual count,
HIGHLIGHT N in formula we divide it by Avogadro’s number,
HIGHLIGHT NA
to get the mole count.
HIGHLIGHT n
The same basic idea is involved when we convert any individual count to a group count.
For example, if you have 36 eggs, you divide it by 12 to get the dozen count.
36 divided by 12 is 3.
36 eggs is equivalent to 3 dozen eggs.
CLICK PAUSE END RECORDING
1 mol
3.0x1022 x
= 0.50 mol
6.02 x 1023
Individual count  / Avogadro’s number  Mole count
n = N / NA

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08