Molar Mass and the Mole  

  1. Writing and Balancing Chemical Reactions
    1. Reactant(s) g Product(s).   Mass does not change during the reaction.  You start and end with the same exact mass.
    2. The # of atoms does not change during the reaction.  The identity of the atoms does not change during the reaction.  If you start with four carbon atoms, you must end with four carbon atoms.
    3. The charge stays the same.  If all the reactants are neutral overall, the products overall must be neutral.  Bonds do change.  Bonds break and form during a chemical reaction.
    4. Law of Conservation of Mass = mass cannot be created or destroyed.  This is why we balance reactions.
      1. Note that H2O means there are 2 H atoms and 1 O atoms bonded together in the molecule while 2 H2O means there are two separate water molecules
      2. Draw pictures to help you count atoms.  Balancing reactions is largely trial and error.  You put numbers in front of the compounds and check to see if that helped you get closer to having the same number of atoms on both sides.  
      3. The first step is to write the reaction with the correct compounds.  Remember that charges determine the formula of the compound so that the compound is neutral, such as NaCl, MgCl2, etc...  Once you determine the correct formula you CAN NOT change the subscripts.  If you need two sodium atoms, don't change NaCl to Na2Cl - there is no such thing.  
      4. We balance chemical reactions by adding coefficients - numbers in from the the compound.  If you need two sodiums, you write 2 NaCl which means you have 2 distinct and separate NaCl units.  
    5. Formation Reactions
      1. In general, element + element g compound where the elements are in their naturally occurring state (solid, liquid or gas) and diatomic or not.
      2. Write and balance the formation reaction between hydrogen and oxygen:  2 H2(g)  +  O2(g)  g  2 H2O
      3. Write and balance the formation reaction between sodium and chlorine:  2 Na(s)  +  Cl2(g)  g  2 NaCl
      4. Other examples
        1. 4 As(s) + 5 O2(g) g 2 As2O5
        2. C(s) + O2(g) g CO2
        3. I2(s) + 7 Cl2(g) g 2 ICl7
        4. 2 Li(s) + Br2(l) g 2 LiBr
        5. 4 Fe(s) + 3 O2(g) g 2 Fe2O3
    6. Combustion Reactions
      1. In general for hydrocarbons, CxHy + O2(g) g CO2(g) + H2O(g)
      2. Write and balance the combustion reaction for C2H4:  C2H4 + 3 O2(g) g 2 CO2(g) + 2 H2O(g)
      3. Write and balance the combustion reaction for C3H8:  C3H8 + 5 O2(g) g 3 CO2(g) + 4 H2O(g)
  2. Molar Mass (MM) (aka Formula Weight, Molecular Weight)
    1. Molar Mass is the sum of all the atomic masses
    2. Examples:  Check yourself.
      1. H2O is 18.0 g/mol
      2. NaCl is 58.5 g/mol
      3. Ca3(PO4)2 is 310.3 g/mol
  3. Avogadro's Number - The Mole  
    1. The mole is just a number like a dozen is 12.
    2. A mole is a large number = 6.02 x 1023.  This number is Avagadro's Number and is given the symbol NA
    3. A mole of something will not weigh the same as a mole of something else, just like a dozen eggs do not weight the same as a dozen cars. 
    4. We measure things by weighing them or counting them.  One dozen = 12 anythings just like one mole = 6.02 x 1023 anythings.  It is convenient to use dozens for cookies, donuts, and eggs.  It is convenient to use moles for atoms and molecules because they are so small.  We cannot actually count out atoms or molecules however.
      1. one dozen donuts = 12 donuts
      2. one mole of donuts = 6.02 x 1023 donuts
      3. one mole of iodine = 6.02 x 1023 atoms of iodine
      4. one mole of water = 6.02 x 1023 molecules of water
      5. one mole of marbles = 6.02 x 1023 marbles (which would fill up the Pacific ocean)
    5. The periodic table shows us the mass of one mole of atoms for each element. (MM)
      1. 6.02 x 1023 carbons atoms = one mole = 12.0 grams thus 12.0 g/mol for carbon
      2. 6.02 x 1023 oxygen atoms = one mole = 16.0 grams thus 16.0 g/mol for oxygen.
      3. 6.02 x 1023 molecules of O2 masses 32.0 grams.  So O2 is 32.0 g/mol
      4. 6.02 x 1023 water molecules = one mole = 18.0 grams thus 18.0 g/mol for water.
    6. Practice Questions.
      1. one mole of Neon has how many atoms?           answer: 6.02 x 1023
      2. 23.0 grams of sodium is how many atoms?               answer:  this is one mole so 6.02 x 1023 atoms
      3. 6.02 x 1023 H atoms weigh how much?                answer: 1.0 gram
      4. one mole of carbon dioxide weighs how much?                answer: 44.0 grams (add up one C and two O)
    7. Molar ratios - Use molar ratios to go between moles of a molecule and moles of its atoms
      1. The ratio of atoms per molecule such as (2 H  / 1 H2O)  is a molar ratio. 
      2. These ratios can be used in calculations such as:
        1. How many moles of H atoms are in 23.0 moles of NH3?              Answer is 23.0 mol NH3 ( 3 H  / 1 NH3 ) = 69.0 mol H atoms
        2. How many moles of Cl atoms are in 4.2 moles of chlorine?             Answer is 4.2 moles Cl2 ( 2 Cl  / 1 Cl2 ) = 8.4 mol Cl atoms
        3. How many moles of Chloride ions are in 4.4 moles of table salt?          Answer is 4.4 mol NaCl (1 Cl-  / 1 NaCl ) = 4.4 mol Cl- ions
    8. Examples: Converting between moles and the number of actual atoms or molecules using NA.
      1. How many atoms are in 2.50 moles of carbon?        Answer 2.50 mol C (6.02 x 1023 atoms/mol) = 1.51 x 1024 C atoms
      2. How many moles are in 5.00 x 1023 N atoms?        Answer 5.00 x 1023 N atoms ( mol / 6.02 x 1023 atoms) = 0.831 mol N
      3. How many molecules are in 4.7 mol NH3?        Answer 4.7 mol NH3 (6.02 x 1023 molecules/mol) = 2.8 x 1024 molecules NH3
    9. Consider this balanced reaction and what it means:  3 H2 (g)  +  N2 (g)  g  2 NH3
      1. 3 molecules of hydrogen gas react with one molecule of nitrogen gas to make 2 molecules of ammonia
      2. 3 moles of hydrogen gas react with one mole of nitrogen gas to make 2 moles of ammonia
      3. 3(2.0 g/mol) + 28.0 g/mol = 2(17.0) g/mol  (Each side weighs 34 grams which follows the Law of Conservation of Mass)

The End           Practice Problems Below

Now we are ready to practice writing and balancing chemical reactions.  Remember, once you have written the reaction and have the correct formulas, you can NOT change the subscripts.  You can only add coefficients in front of the compounds to balance the reaction. 

  1. Balance the following formation reaction:  Ca (s) + I2(s) g CaI2
  2. Balance the following formation reaction:  Na (s) + Br2(l) g NaBr
  3. Write and balance the formation reaction for magnesium and chlorine.
  4. Write and balance the formation reaction for potassium and oxygen.
  5. Write and balance the formation reaction for calcium and oxygen.
  6. Write and balance the formation reaction for phosphorus pentachloride.
  7. Balance the following combustion reaction:  C4H8 + O2(g) g CO2(g) + H2O(g)
  8. Balance the following combustion reaction:  C6H14 + O2(g) g CO2(g) + H2O(g)
  9. Write and balance the combustion reaction for C6H6.
  10. Write and balance the combustion reaction for C3H6.
  11. Balance the following  reaction:  CaI2 + Na2SO4 g CaSO4 + NaI
  12. Balance the following  reaction:  MgSO4 + KNO3 g Mg(NO3)2 + K2SO4

Answers

  1. already balanced
  2. 2 Na (s) + Br2(l) g 2 NaBr
  3. Mg (s) + Cl2(g) g MgCl2
  4. 4 K (s) + O2(g) g 2 K2O
  5. 2 Ca (s) + O2(g) g 2 CaO
  6. 2 P(s) + 5 Cl2(g) g 2 PCl5
  7. C4H8 + 6 O2(g) g 4 CO2(g) + 4 H2O(g)
  8. 2 C6H14 + 19 O2(g) g 12 CO2(g) + 14 H2O(g)
  9. 2 C6H6 + 15 O2(g) g 12 CO2(g) + 6 H2O(g)
  10. 2 C3H6 + 9 O2(g) g 6 CO2(g) + 6 H2O(g)
  11. CaI2 + Na2SO4 g CaSO4 + 2 NaI
  12. MgSO4 + 2 KNO3 g Mg(NO3)2 + K2SO4

Practice Problems

  1. How many atoms are in 4.00 moles of carbon?
  2. How many moles are in 1.25 x 1025 atoms of neon?
  3. How many molecules are in 2.99 moles of ammonia?
  4. How many moles are in 5.25 x 1024 units of lithium sulfide?
  5. How many units are in 4.24 moles of sodium chloride?
  6. How many H atoms are in 8.35 moles of water?
  7. How many O atoms are in 1.44 moles of magnesium sulfate?
  8. If I have 3.45 x 1025 H atoms in a pure CH4 sample, how many moles of CH4 are present?
  9. What is the molar mass of ReH7, C6H6, KNO3, CuCl2, H2O2, Ne, CaSO4, Mg3(PO4)2, (NH4)2CO3, Na?
  10. 6.02 x 1023 Cl atoms weigh how much? 
  11. One mole of carbon dioxide contains how many molecules? 
  12. How many moles are in 7.25 x 1026 molecules oxygen gas?  

ANSWERS

  1. 4.00 mol C (6.02 x 1023 atoms / mol) = 2.41 x 1024 C atoms
  2. 1.25 x 1025 atoms Ne ( mol / 6.02 x 1023 atoms) = 20.8 mol Ne
  3. 2.99 mol NH3 (6.02 x 1023 molecules / mol) = 1.80 x 1024 molecules NH3
  4. 5.25 x 1024 units Li2S (mol / 6.02 x 1023 units) = 8.72 mol Li2S
  5. 4.24 mol NaCl (6.02 x 1023 units / mol) = 2.55 x 1024 units NaCl
  6. 8.35 mol H2O ( 2 H / 1 H2O) (6.02 x 1023 atoms / mol) = 1.01 x 1025 H atoms
  7. 1.44 mol MgSO4 ( 4 O / MgSO4) (6.02 x 1023 atoms / mol) = 3.47 x 1024 O atoms
  8. 3.45 x 1025 H atoms ( 1 CH4 molecule/ 4 H atoms) (mol / 6.02 x 1023 molecules) = 14.3 mol CH4
  9. ReH7-193.2 g/mol, C6H6-78.0 g/mol, KNO3-101.1 g/mol, CuCl2-134.5 g/mol, H2O2-34.0 g/mol,  Ne-20.2 g/mol, CaSO4-136.2 g/mol, Mg3(PO4)2-262.9 g/mol, (NH4)2CO3-96.0 g/mol,  Na-23.0 g/mol
  10. 35.5 grams - cause it is one mole - just look at the Periodic Table
  11. 6.02 x 1023
  12. 7.25 x 1026 molecules O2 (mol/6.02 x 1023 molecules) = 1.20 x 103 mol O2