Understanding the Concept of Molar Mass in Chemistry

Explore the concept of molar mass in chemistry, including the definition of the mole, Avogadro's number, calculations for molar mass of elements and compounds, and examples of determining molar mass. Discover how to find the molar mass of various compounds through practical examples.

• Chemistry
• Molar Mass
• Avogadros Number
• Compounds
• Elements

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1. The Mole (Chapter 10) Dr. Walker

2. The Mole in Chemistry Not this!! Not this!!!

3. The Mole The mole is a counting unit 1 pair = 2 1 dozen = 12 1 mole = 6.022 x 1023atoms This number is known as Avogadro s number after Italian chemist Amedeo Avogadro

4. The mole One mole can be equal to three different things 6.022 x 1023atoms The atomic mass in grams of an atom or compound 22.4 L (volume) of any gas at 25 oC and 1 atm pressure (known as STP)

6. Molar Mass The atomic mass of any element is the weighted average of all isotopes of an element This is also the molar mass of the element Examples H = 1.01 g/mole O = 16.00 g/mole C = 12.01 g/mole P = 30.97 g/mole Cl = 35.45 g/mole

7. Molar Mass The molar mass of a compound is the sum of all atomic weights of all atoms in the compound!

8. Molar Mass Examples For compounds H2O H = 1.01 g/mole x 2 O = 16.00 g/mole x 1 =18.02 g/mole

9. Molar Mass Examples Another example CH4 C = 12.01 g/mole = 12.01 H = 1.01 g/mole x 4 = 4.04 16.05 g/mole

10. Molar Mass Examples Another Example C6H12O6 C = 12.01 g/mole x 6 = 72.06 H = 1.01 g/mole x 12 = 12.12 O = 16.00 g/mole x 6 = 96.00 180.18 g/mole

11. Molar Mass Examples Another Example Polyatomics Pb(SO4)2 Pb = 207.20 S = 32.07 g/mole x 2 = 64.14 O = 16.00 g/mole x 8 = 128.00 399.34 g/mole Notice you must multiply the subscript inside the parentheses by the number outside to get the right number of atoms

12. Practice Find the molar mass of the following compounds: LiOH SO2 Mg(NO3)2 Al2(CO3)3 C9H8O4 C8H10N4O2 C22H30N6O4S

13. Practice Find the molar mass of the following compounds: LiOH 6.94 + 16.00 + 1.01 = 23.95 g/mole SO2 32.06 + 2(16.00) = 64.06 g/mole Mg(NO3)2 24.31 + 2(14.01) + 6(16.00) = 148.33 g/mole Al2(CO3)3 2(26.98) + 3(12.01) + 9(16.00) = 233.99 g/mole C9H8O4 9(12.01) + 8(1.01) + 4(16.00) = 180.17 g/mole C8H10N4O2 8(12.01) + 10(1.01) + 4(14.01) + 2(16.00) = 194.22 g/mole C22H30N6O4S 22(12.01) + 30(1.01) + 6(14.01) + 4(16.01) + 1(32.06) = 474.68 g/mole

14. Mass-Mole Conversions Convert from mass to mole (or vice versa) using the molar mass of a given element or compound Use dimensional analysis (t-square)

15. Mass-Mole Conversions How many grams are in 2.5 moles NH3?

16. Mass-Mole Conversions How many grams are in 2.5 moles NH3? Find molar mass of NH3 N = 14.01 g/mole x 1 = 14.01 g/mole H = 1.01 g/mole x 3 = 3.03 g/mole Molar mass = 17.04 g/mole Multiply # of moles by molar mass 2.5 moles NH3 17.04 g NH3 = 42.6 g NH3 1 mole NH3

17. Mass-Mole Conversions How many moles are contained in 100 g C3H8?

18. Mass-Mole Conversions How many moles are contained in 100 g C3H8? Find molar mass C = 12.01 g/mole x 3 = 36.03 g/mole H = 1.01 g/mole x 8 = 8.08 g/mole 44.11 g/mole Divide # of grams given by molar mass 100 g C3H8 1 mole C3H8 = 2.27 moles C3H8 44.11 g C3H8

19. Mass-Mole Conversions How many grams are present in 3.1 moles of lithium carbonate?

20. Mass-Mole Conversions How many grams are present in 3.1 moles of lithium carbonate? Formula for lithium carbonate = Li2CO3 Li = 6.94 g/mole x 2 = 13.88 g/mole C = 12.01 g/mole x 1 = 12.01 g/mole O = 16.00 g/mole x 3 = 48.00 g/mole 73.89 g/mole Multiply number of moles by molar mass 3.1 moles Li2CO3 73.89 g Li2CO3 = 229.06 g Li2CO3 1 mole Li2CO3

21. Mole-Particle conversions Can go to or from moles from particles Atoms in an element Molecules in a covalent compound You may see other terms like ions, formula units 1 mole = 6.022 x 1023 particles is the conversion factor that you use

22. Mole-Particle Conversions How many atoms are in 1.5 moles of iron?

23. Mole-Particle Conversions How many atoms are in 1.5 moles of iron? 1.5 moles iron 6.022 x 1023 atoms iron = 9.03 x 1023 atoms iron 1 mole iron

24. Mole-Particle Conversions How many moles are in 2.4 x 1024 molecules of water?

25. Mole-Particle Conversions How many moles are in 2.4 x 1024 molecules of water? 2.4 x 1024 molecules H2O 1 mole H2O = 3.98 moles H2O 6.022 x 1023 molecules H2O

26. Mole-Particle Conversions It doesn t matter what atom or compound is being converted For these conversions, you are always using 1 mole = 6.022 x 1023 particles as your conversion factor!!

27. Mole-Volume Conversions At a specific set of conditions (known as Standard Temperature and Pressure or STP), 1 mole of any gas = 22.4 L of volume We can convert volume to moles and vice versa

28. Example How many liters of O2 are contained in 5.0 moles?

29. Example How many liters of O2 are contained in 5.0 moles? 5.0 moles O2 22.4 L O2 = 112 L O2 1 mole O2

30. Example How many moles are contained in a 3.7 L sample of N2?

31. Example How many moles are contained in a 3.7 L sample of N2? 3.7 L N2 1 mole N2 = 0.17 moles N2 22.4 L N2

32. Multistep Conversions We know that the mole is equal to multiple numbers ( Avogadro s # = 1 mole = molar mass = 22.4 L) Given this fact, we can convert from particles to mass using the mole Hint: If you look at the problem and the word mole is absent, it is multistep!!

33. Multistep Example How many grams are present in a 10 L sample of H2?

34. Multistep Example How many grams are present in a 10 L sample of H2? Convert from liters to moles 1 mole H2 10 L H2 22.4 L H2 Convert from moles to grams

35. Multistep Example How many grams are present in a 10 L sample of H2? Convert from liters to moles Convert from moles to grams 1 mole H2 2.02 g H2 10 L H2 = 0.90 g H2 22.4 L H2 1 mole H2

36. Example How many molecules are contained in 4.5 g H2O?

37. Example How many molecules are contained in 4.5 g H2O? Step 1 Convert from grams to moles 4.5 g H2O 1 mole H2O 18 g H2O

38. Example How many molecules are contained in 4.5 g H2O? Step 2 Convert from moles to particles 0.25 moles H2O 6.022 x 1023 molecules H2O 1 mole H2O 4.5 g H2O = 1.50 x 1023 molecules H2O 1 mole H2O 18 g H2O

39. Vocab/Skills Vocabulary Molar Mass Avogadro s number Skills Convert to and from moles using mass/particles/volumes using dimensional analysis