1. How many atoms of carbon are in a 2.50 mol sample of carbon?
QUESTION:
How many atoms of carbon are in a 2.50 mol sample of carbon?
A x1024, B. 30.0, C. 4.15x10-22
Definition: 1 mol = x1023
Individual count
6.022 x 1023 =
Mole count
1 mol
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
We should be able to eliminate choice C right away.
CROSS OUT C
4.15x10-22 is a number less than one. How can you have less than one atom?
We should also be able to eliminate choice B.
One mole corresponds to a count of about 602 billion trillion...
Thirty is way less than this number.
which is even less than what is being asked .... the number in two-and-a-half moles...
So choice A must be correct. Let’s verify.
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 atoms, you also have zero moles of atoms.
If you have twice as many atoms, the moles of atoms would also double.
When two quantities are directly proportional, their ratio is constant.
In other words, we can say that the ratio of
the Individual atom count
CLICK
to the group count or mole count
CLICK CLICK
is equal to
the ratio of 6.022x1023
to 1 mole.
We can rearrange this equation to solve for the individual atom count. Essentially, what we would end up doing is
multiply the given mole count, 2.50 mol
by the ratio of Avogadro’s number to 1 mole.
CLICK CLICK CLICK
The ratio serves as a conversion factor.
SHOW CANCELLATION OF UNITS
Note that we rounded our value for Avogadro’s number to four significant digits.
HIGHLIGHT 6.022
There’s no need to use more because our less precise term has only 3 significant digits.
CALLOUT “3 sig.figs.” pointing to 2.50
The answer we get should be rounded off to three significant figures... and it’s equal to 1.51x1024
TO summarize,
CLICK if we want to convert a mole count to an individual count,
we simply multiply the mole count by Avogadro’s number.
CLICK If lower case n is the mole count,
HIGHLIGHT n in formula we multiply it by Avogadro’s number,
HIGHLIGHT NA
to get the individual count.
The same basic idea is involved when we convert dozens to an individual count.
IF we have two dozen eggs, we multiply 2 by the 12 to get the individual count of 24 eggs.
CLICK PAUSE END RECORDING
2.50 mol
6.022 x 1023 x
= 1.51x1024
1 mol
Mole count  x Avogadro’s number  Individual 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