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. 2.0 mol, B. 1.06x10-21 mol, C. 0.50 mol Definition: 1 mol = 6.02213x1023 Mole count 1 mol = Individual count 6.02213 x 1023 How many atoms of carbon are in a 2.50 mol sample of carbon? A. 1.51x1024, 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 6.02213x1023 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

Video ID: 4-3-2 © 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