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Understanding The Mole in Chemistry: Avogadro's Number & Molecular Weight

Learn about "The Mole" in chemistry – Avogadro’s Number, calculating molecular weight, molar mass, converting moles to grams, significant figures. Understand the fundamental concepts easily!

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Understanding The Mole in Chemistry: Avogadro's Number & Molecular Weight

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  1. The Mole Pay Attention this is really important!

  2. What is “The Mole?” It’s not this…

  3. Or this…

  4. And we’re not talking about food either…

  5. We’re talking about this…

  6. The Mole Defined • The mole is the standard method in chemistry for communicating how much of a substance is present. • Here is how the International Union of Pure and Applied Chemistry (IUPAC) defines "mole:"

  7. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. This is the fundamental definition of what one mole is. One mole contains as many entities as there are in 12 grams of carbon-12 (or 0.012 kilogram).

  8. In one mole, there are 6.022 x 1023 atoms. Here's another way: there are 6.022 x 1023 atoms of carbon in 12 grams of carbon-12 Holy Smokes! That is a lot of zeroes…and that number actually has a name.

  9. Avogadro’s Number • Never call it Avocado’s Number. Remember, we’re not talking about guacaMOLE here, just the mole.

  10. 6.022 x 1023 is such an important number that it has its own name and symbol, N. • Named Avogadro’s Number in honor of Italian chemist who made critical contributions to measuring atomic weights.

  11. The Mole Applied • In one mole of ANYTHING there are 6.022 x 1023 entities. So… One mole of donuts contains 6.022 x 1023 donutsOne mole of carbon atoms contains 6.022 x 1023 of carbon atomsOne mole of water molecules contains 6.022 x 1023 of H2O molecules.

  12. Molecular Weight (Mass) • Molecular Weight = weight in atomic mass units of all the atoms in a given formula. • Symbol = was and is most commonly amu but not the most correct symbol is u. • Why do we need to know molecular weight? It is needed in order to know how many grams are in one mole of a substance.

  13. Four Steps to Calculating a substance’s molecular weight: • Determine how many atoms of each different element are in the formula. • Look up the atomic weight of each element in a periodic table • Multiply step one times step two for each element • Add the results of step three together and round off as necessary.

  14. Practice • KCl1. K = 1 Cl = 12. K = 39.10 Cl = 35.453. 1(39.10) + 1(35.45) = 4. 74.55 amu represents the molecular weight of KCl

  15. More Practice • H2O2 • H = 2 O = 2 • H = 1.00 O = 15.99 • 2(1) + 2(15.99) = • 2+32 = 34 amu

  16. Molar Mass • Counting atoms or molecules is very difficult since they are so small BUT you can “count” atoms or molecules by weighing large amounts of them on a balance. • When we weigh one mole of a substance on a balance, this is called “molar mass” and has the units g/mol (grams per mole).

  17. Molar Mass • A molar mass is the weight in grams of one mole • One mole contains 6.022 X 1023 entities.Therefore, a molar mass is the mass in grams of 6.022 X 1023 entities

  18. How to Calculate Molar Mass • How do YOU calculate molar mass? The molar mass IS the molecular weight (!) in grams (not amu’s) • So You calculate the molecular weight and stick the unit “g/mol” after the number and that is the molar mass for the substance in question.

  19. Example of Molar Mass • Al(NO3)3 • (1xAl)+(3xN)+(9xO)  (1x 26.98)+(3x14.007) + (9x 16.00) = 213.00 g/mol • 213 grams is the mass of one mole of aluminum nitrate • 213 trams of aluminum nitrate contains 6.022 x 1023 entities of Al(NO3)3

  20. Practice AlCl3

  21. Converting Moles to Grams • The mole is the standard measurement amount in chemistry and when substances react with one another they do so in ratios of moles. Problem is…balances give readings in grams, not moles! • So, when comparing one substance to another using moles….we have to convert moles to grams since this is the unit we measure from balances.

  22. The Steps • 3 steps to converting moles of a substance to grams: • Determine how many moles are given in the problem • Calculate the molar mass of the substance • Multiply step one by step two.

  23. The Equation • The equation looks like this: • Grams of the substance = molar mass of the substance in gramsmoles of the substance one mole • You replace the “grams of the substance” with X because this is what you are solving for.

  24. Example Problem Problem: Calculate how many grams are in 0.700 moles of H2O2 • Determine how many moles you have ---- 0.700 moles • Determine the molar mass of the substance ---- H2O2 has a molar mass of 34.0146 g/mol • Multiply the moles given by the substance’s molar mass:0.700 mole x 34.0146 g/mol = 23.8 grams

  25. Significant Figures • The answer of 23.8 trams has been rounded to three significant figures because the 0.700 value had the least number of significant figures in the problem. • Using the equation – this problem would look like: x = 34.0146 g 0.700 mol 1.00 mol(then cross multiply and divide to solve for x)

  26. More practice Problem #2: Convert 2.50 moles of KClO3 to grams.

  27. Converting Grams to Moles The mole is the standard measurement of amount. However, remember that balances do NOT give readings in moles, they give readings in grams. Oh NO!! Steps to convert grams of a substance to moles • Determine how many grams are given in the problem • Calculate the molar mass of the substance. • Divide step one by step two.

  28. This might look familiar… • Here is the equation: • Grams of the substance = molar mass of the substance in gramsmoles of the substance one mole • Where the “moles of the substance” will be the unknown (x) and what you solve for.

  29. Let’s Practice • Convert 25.0 grams of KMnO4 to moles • Determine how many grams are present --- 25.0 grams • Determine the molar mass of the substance --- KMnO4 is 158.034 g/mol • Divide the grams given by the substance’s molar mass25.0 g = 0.158 mol158.034 g/mol

  30. If this problem were set up like the proportion, it would look like this: • 25.0 g = 158.034 g x 1.00 mol • Then, you cross-multiply and divide to solve for X.

  31. And one more practice problem… Calculate how many moles are in 17.0 grams of H2O2

  32. Calculating The Mass of One Molecule • When calculating the mass of one molecule, do the following: • Calculate the molar mass of the substance • Divide it by Avogadro's Number • Make sure you have a periodic table and a calculator handy. • the technique to calculate the mass of one atom of an element is exactly the same as for calculating the mass of one molecule of a compound.

  33. Practice problem #1 • What is the mass of one molecule of H2O? • Step #1 - the molar mass of water is 18.015 g/mol. This was calculated by multiplying the atomic weight of hydrogen (1.008) by two and adding the result to the weight for one oxygen (15.999). • Please remember that you need the molar mass first when trying to find the mass of one molecule.

  34. Step #2 - divide the substance's molar mass by Avogadro's Number. • 18.015 grams/mol ÷ 6.022 x 1023 mol¯1 = 2.992 x 10¯23 gramsNote that the final answer has been rounded to four significant figures (from 2.9915 - note use of rounding with five rule) and that the unit of mole cancels.

  35. Practice #2 • calculate the mass (in grams) of one molecule of CH3COOH • The molar mass of CH3COOH is 60.06 g/mol. The solution is: • 60.06 g/mol ÷ 6.022 x 1023 mol¯1 = 9.973 x 10¯23 g

  36. Acknowledgements • Thanks to the ChemTeam for much of the valuable information on this power point.

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