Stoichiometric

1
Stoichiometric Calculations
2
Chocolate Chip Cookies

• 2.25 cups flour
• 8 Tbsp butter
• 0.5 cups shortening
• 0.75 cups sugar
• 0.75 cups brown sugar
• 1 tsp salt
• 1 tsp baking soda
• 1 tsp vanilla
• 0.5 cups Egg Beaters
• 2 cups chocolate chips

3
Chocolate Chip Cookies

• 2.25 cups flour
• 8 Tbsp butter
• 0.5 cups shortening
• 0.75 cups sugar
• 0.75 cups brown sugar
• 1 tsp salt
• 1 tsp baking soda
• 1 tsp vanilla
• 0.5 cups Egg Beaters
• 2 cups Chocolate chips

4
Chocolate Chip Cookies

• 2.25 flour
• 8 butter
• 0.5 shortening
• 0.75 sugar
• 0.75 brown sugar
• 1 salt
• 1 baking soda
• 1 vanilla
• 0.5 Egg Beaters
• 2 Chocolate chips

5
Chocolate Chip Cookies

• 2.25 cups flour
• 8 Tbsp butter
• 0.5 cups shortening
• 0.75 cups sugar
• 0.75 cups brown sugar
• 1 tsp salt
• 1 tsp baking soda
• 1 tsp vanilla
• 0.5 cups Egg Beaters
• 2 cups Chocolate chips

6
Get on with it!
What does this have to do with CHEMISTRY?
7
2.25 cups flour 8 Tbsp butter 0.5 cups
shortening 0.75 cups sugar 0.75 cups brown
sugar 1 tsp salt 1 tsp baking soda 1 tsp
vanilla 0.5 cups Egg Beaters
2 cups Chocolate chips
unit
substance
coefficient
(a synthesis reaction)
8
Welcome to STOICHIOMETRY
9

• The coefficients in chemical equations represent
  numbers of molecules or formula units, not masses
  of molecules or formula units.
• When a reaction is to be run in a laboratory or
  chemical plant, the amount of substances needed
  cannot be determined by counting molecules
  directly.
• Counting is always done by weighing.
• We will see how chemical equations can be used to
  determine the masses of reacting chemicals.

10

• 3NaOH (aq) H3PO4 (aq) Na3PO4 (aq) 3 H2O
• In the above reaction, the coefficients give us
  the ratios by moles of the reactants and
  products.
• The coefficients tell us that to make 1 mol of
  Na3PO4 from 1 mol of H3PO4, we must also use 3
  mol of NaOH.
• We dont however, have to carry out the reaction
  with these actual numbers of moles.
• Whatever quantities we choose must be in the
  proportions set by the coefficients.
• Regardless of the scale of the reaction, the
  coefficients of a chemical equation give the
  ratio in which the moles of one substance react
  with or produce moles of another.

11

• 3NaOH (aq) H3PO4 (aq) Na3PO4 (aq) 3 H2O
• We can look at this equation as a calculating
  tool, because its coefficients give us
  stoichiometric equivalencies between the
  substances involved.
• For example, from the above equation we can
  generate the following equivalencies
• 3 mol NaOH ? 1 mol H3PO4
• 3 mol NaOH ? 1 mol Na3PO4
• 3 mol NaOH ? 3 mol H2O
• 1 mol H3PO4? 1 mol Na3PO4
• 1 mol H3PO4? 3 mol H2O
• Any of these can be used to construct conversion
  factors called mole ratios for stoichiometric
  calculations.

12

• Example How many moles of sodium phosphate,
  Na3PO4, can be made from 0.240 mol of NaOH by the
  following reaction?

3NaOH (aq) H3PO4 (aq) Na3PO4 (aq) 3H2O
13

• In practical work, a chemist is often confronted
  by a question such as the following.
• If I start with so many grams of reactant A, how
  many grams of reactant B ought I use, and how
  many grams of a particular product should be
  produced?
• Notice that the question concerns grams, not
  moles, for the practical reason that masses in
  grams are delivered by laboratory balances.
• The coefficients of the desired reaction,
  however, know nothing about grams, only about
  relative numbers of moles.
• If we know two facts, namely, the balanced
  equation and the mass of any substance in it, we
  can calculate the required or expected mass of
  any other substance in the equation.

14

• Example Portland cement is a mixture of the
  oxides of calcium, aluminum, and silicon. The
  raw material for its calcium oxide is calcium
  carbonate, which occurs as the chief component of
  a natural rock, limestone. When calcium
  carbonate is strongly heated it decomposes by the
  following reaction. One product CO2, is driven
  off to leave the desired CaO as the only other
  product.
• CaCO3 (s) CaO (s) CO2 (g)
• A chemistry student is to prepare 1.50 x 102 g of
  CaO in order to test a particular recipe for
  portland cement. How many grams of CaCO3 should
  be used, assuming that all will be converted?

heat
15

• Calculating Masses of Reactants and Products in
  Chemical Reactions
• Balance the equation for the reaction.
• Convert the known mass of the reactant or product
  to moles of that substance.
• Use the balanced equation to set up the
  appropriate mole ratios.
• Use the appropriate mole ratios to calculate the
  number of moles of the desired reactant or
  product.
• Convert from moles back to grams if required by
  the problem.

16

• Example One of the most spectacular reactions
  of aluminum, the thermite reaction, is with iron
  oxide, Fe2O3 , by which metallic iron is made.
  So much heat is generated that the iron forms in
  the liquid state. The equation is
• Al (s) Fe2O3 (s) Al2O3 (s) Fe (l)
• A certain welding operation, used over and over,
  requires that each time at least 86.0 g of Fe be
  produced. What is the minimum mass in grams of
  Fe2O3 that must be used for each operation?
  Calculate also how many grams of aluminum are
  needed.

17
The Wisdom of Gallagher
Why are there Interstate Highways in Hawaii?
Why are there floatation devices under plane
seats instead of parachutes?
Why do we drive on parkways and park on
driveways?
Why do hot dogs come ten to a package and hot dog
buns only eight?
18
Hot Dogs in the News
Takeru Kobayashi of Japan downed 44½ hot dogs
in 12 minutes.
One hot dog one hot dog one bun.
WHAT IF
Mr. Kobayashi didnt do his math correctly. He
bought 5 packs of hot dogs (10 per package) and 5
packs of hot dog buns (8 per package). How many
hot dogs (according to the official formula)
could he have eaten?
Source CNN.com
19
Hot Dogs in the News
One hot dog one hot dog one bun.
WHAT IF
Mr. Kobayashi didnt do his math correctly. He
bought 5 packs of hot dogs (10 per package) and 5
packs of hot dog buns (8 per package). How many
hot dogs (according to the official formula)
could he have eaten?
Excess
5 hot dog packs
10 hot dogs
50 hot dogs
x

1 hot dog pack
Limiting
40 possible hot dogs
5 bun packs
8 buns
40 buns
x

1 bun pack
Source CNN.com
20
Lets Revisit the Cookies (again)
For 1 batch

• In my pantry, I have
• 5.5 cups of flour
• 16 Tbsp of butter
• lots of everything else

• 2.25 cups flour
• 8 Tbsp butter
• 0.5 cups shortening
• 0.75 cups sugar
• 0.75 cups brown sugar
• 1 tsp salt
• 1 tsp baking soda
• 1 tsp vanilla
• 0.5 cups Egg Beaters
• 12 oz. Chocolate chips

How many batches of cookies can I make?
21
Lets Revisit the Cookies (again)
For 1 batch
How many batches of cookies can I make?
5.5 cups

• 2.25 cups flour
• 8 Tbsp butter
• 0.5 cups shortening
• 0.75 cups sugar
• 0.75 cups brown sugar
• 1 tsp salt
• 1 tsp baking soda
• 1 tsp vanilla
• 0.5 cups Egg Beaters
• 12 oz. Chocolate chips

16 Tbsp
EXCESS
1 batch cookies
5.5 c flour
x

2.25 c flour
2.4 batches
1 batch cookies
16 Tbsp butter
x

8 Tbsp butter
2.0 batches
22
Now I Want to Bake a Cake!
But do I have all the ingredients I need?
How much flour do I have left after baking all
those cookies?
SOME FLOUR LEFT OVER
GONE!
2.25 cups flour
4.5 cups flour used
2.0 batches
x

1 batch cookies
5.5 cups 4.5 cups 1.0 cups left
23
Limiting Reactant Calculations

• Many times a chemist will mix reactants in a mole
  ratio that does not agree with the coefficients
  of the equation.
• Some reactions proceed better when one reactant
  is in stoichiometric excess, for example.
• One such reaction is the preparation of ammonia,
  NH3, from its elements.
• N2 (g) 3H2 (g) 2NH3 (g)
• Suppose a chemist mixed 1.00 mol of N2 with 5.00
  mol of H2. What is the maximum number of moles
  of product that could form?

24
N2 (g) 3H2 (g) 2NH3 (g)

• Note the coefficients tell us that 1 mol of N2
  consumes 3 mol of H2.
• 1 mol N2 ? 3 mol H2
• But 5 mol of H2 was used, not 3, so there will be
  2 mol of H2 left over.
• Once the 1 mol of N2 taken is consumed, no
  additional NH3 can form.
• Therefore, the reactant that is completely
  consumed limits the amount of product that forms,
  so it is called the limiting reactant.
• In this reaction, N2 is the limiting reactant, it
  limited the amount of NH3 that was formed.

25

• Example In an industrial process for making
  nitric acid, the first step is the reaction of
  ammonia with oxygen at high temperature in the
  presence of a platinum gauze. Nitrogen monoxide
  forms as follows
• 4NH3 5O2 4NO 6H2O
• How many grams of nitrogen monoxide can form if a
  mixture of 30.00 g of NH3 and 40.00 g of O2 is
  taken initially? How many grams of the excess
  reactant is left over?

26
Theoretical Yield and Percentage Yield
27

• In most experiments designed for chemical
  synthesis, the amount of a product obtained falls
  short of the calculated maximum amount.
• Losses occur for several reasons.
• Some are mechanical, such as materials sticking
  to glassware.
• But one of the most common causes of obtaining
  less than the stoichiometric amount of a product
  is the occurrence of a competing reaction.
• It produces a by product, a substance made by a
  reaction that competes with the main reaction.

28

• The actual yield of desired product is simply how
  much is obtained or isolated, stated in mass
  units.
• The theoretical yield is the amount of product
  formed when the limiting reactant is completely
  consumed and no losses occur.
• When less than the theoretical yield of product
  is obtained, chemists generally calculate the
  percentage yield or percent yield to describe how
  well the reaction went.
• The percent yield is the actual yield calculated
  as a percentage of the theoretical yield.

29

• Example A chemist set up a synthesis of
  phosphorus trichloride by mixing 12.0 g P with
  35.0 g Cl2 and obtained 42.4 g of PCl3. Calculate
  the percent yield of this compound. The equation
  for the main reaction is
• 2P (s) 3Cl2 (g) 2PCl3 (l)