Just Follow the RecipeJust Follow the Recipe

Introduction to Reaction Introduction to Reaction StoichiometryStoichiometry

Working with RecipesWorking with Recipes

A recipe tells the A recipe tells the “relative” amounts “relative” amounts of various of various componentscomponents

The amounts are The amounts are given in prescribed given in prescribed unitsunits

If you change the If you change the unit system you unit system you mess up the recipemess up the recipe

Here’s a RecipeHere’s a Recipe Chocolate chip Chocolate chip

cookiescookies 3/4 cup sugar3/4 cup sugar

• 3/4 cup packed brown • 3/4 cup packed brown sugarsugar• 1 cup butter, softened• 1 cup butter, softened• 2 large eggs,beaten• 2 large eggs,beaten• 1 teaspoon vanilla extract• 1 teaspoon vanilla extract• 2 1/4 cups all-purpose • 2 1/4 cups all-purpose flourflour• 1 teaspoon baking soda• 1 teaspoon baking soda• 3/4 teaspoon salt• 3/4 teaspoon salt• 2 cups semisweet • 2 cups semisweet chocolate chipschocolate chips• if desired, 1 cup chopped • if desired, 1 cup chopped pecanspecans

How do the amounts of How do the amounts of sugar and brown sugar sugar and brown sugar compare?compare?

If I used 3 cups of sugar If I used 3 cups of sugar how much brown sugar?how much brown sugar?

How do the amounts of How do the amounts of flour and sugar flour and sugar compare?compare?

If I used 3 cups of sugar If I used 3 cups of sugar how much flour do I how much flour do I need?need?

What if I used 2 lbs of What if I used 2 lbs of sugar?sugar?

How could I deal with the 2 lbs of How could I deal with the 2 lbs of sugar dilemma?sugar dilemma?

The recipe calls for The recipe calls for ¾ cup of sugar¾ cup of sugar

To “work” the To “work” the recipe we’ll have to recipe we’ll have to convert lbs of convert lbs of sugar to cupssugar to cups

A conversion factor A conversion factor (or definition) is (or definition) is neededneeded

Let’s say that 1 cup Let’s say that 1 cup of sugar = 0.25 lbof sugar = 0.25 lb

Steps to follow:Steps to follow: Convert 2 lbs to Convert 2 lbs to

cups cups 2 lbs2 lbs = = XX0.25 lbs 1 cup0.25 lbs 1 cupX = 8 cupsX = 8 cups Work recipeWork recipe8 cups S8 cups S = = XX0.75cup S 2.25cup F0.75cup S 2.25cup FX = 24 cups of flourX = 24 cups of flour

What Does It Mean to Follow the What Does It Mean to Follow the Recipe?Recipe?

The amounts must The amounts must be measured in the be measured in the units indicated by units indicated by the recipethe recipe

The amounts must The amounts must stay in the recipes stay in the recipes proportion or ratioproportion or ratio

You have to use You have to use the correct the correct ingredients and ingredients and conditionsconditions

Let’s try an example with coinsLet’s try an example with coins

Given data regarding Given data regarding coin massescoin masses 1 doz Q = 68 g1 doz Q = 68 g 1 doz D = 28 g1 doz D = 28 g 1 doz N = 60 g1 doz N = 60 g

Given data regarding Given data regarding coin values coin values (recipes)(recipes)

1 doz Q = 2.5 doz D1 doz Q = 2.5 doz D 1 doz Q = 5 doz N1 doz Q = 5 doz N 1 doz D = 2 doz N1 doz D = 2 doz N

Some examples of using recipesSome examples of using recipes

969 grams of quarters are equal in 969 grams of quarters are equal in value to what mass of dimes?value to what mass of dimes?

Steps:Steps: 1) convert mass of quarters to dozens of 1) convert mass of quarters to dozens of

quarters. (Why?)quarters. (Why?) 2) Use the “recipe” of 2.5 doz D = 1 doz 2) Use the “recipe” of 2.5 doz D = 1 doz

QQ 3) Convert dozens of dimes to grams3) Convert dozens of dimes to grams

And, the answer is….And, the answer is….

You try this one…You try this one…

3060 grams of 3060 grams of nickels have the nickels have the same value as what same value as what mass of dimes?mass of dimes?

Do the same three Do the same three stepssteps Get into recipe unitsGet into recipe units Use recipeUse recipe Go back to Go back to

measuring unitsmeasuring units And, the answer is…And, the answer is…

This same technique works for This same technique works for chemical reactionschemical reactions

The recipe calls for The recipe calls for numbernumber of atoms of atoms or molecules or ionsor molecules or ions

2 Al + 3 CuCl2 Al + 3 CuCl22 2 AlCl 2 AlCl33 + 3 Cu + 3 Cu means 2 aluminum atoms for each 3 means 2 aluminum atoms for each 3 cupric chloride moleculescupric chloride molecules

But, we measure in gramsBut, we measure in grams So, we use the mole because of its So, we use the mole because of its

two meanings two meanings (see link above)(see link above)

A Chemistry TaleA Chemistry Tale

What mass of lead(II) phosphate can What mass of lead(II) phosphate can be made from 2.00 grams of sodium be made from 2.00 grams of sodium phosphate with excess lead(II)nitrate?phosphate with excess lead(II)nitrate?

Reaction: Reaction: 3 Pb(NO3 Pb(NO33))22 (aq) + 2 Na (aq) + 2 Na33POPO44 (aq) (aq) Pb Pb33(PO(PO44))22 + 6 + 6

NaNONaNO33(aq)(aq)

2.00g2.00g (3)get (3)get gramsgrams

(1) find moles (2)find moles(1) find moles (2)find moles

Find moles Find moles

2.00 grams of sodium phosphate = ? 2.00 grams of sodium phosphate = ? molesmoles

2.00 g2.00 g = = XX

164 g 1 mol164 g 1 mol

X = 0.01220 mol NaX = 0.01220 mol Na33POPO44

Relate mols of NaRelate mols of Na33POPO44 to mols of to mols of

PbPb33(PO(PO44))22

1 Pb1 Pb33(PO(PO44))22 = = XX

2 Na2 Na33POPO44 0.01220 mols Na 0.01220 mols Na33POPO44

X = 0.006098 mols PbX = 0.006098 mols Pb33(PO(PO44))22

Moles back to gramsMoles back to grams

0.006098 mols Pb0.006098 mols Pb33(PO(PO44))22 = = XX 1 mol Pb1 mol Pb33(PO(PO44))22 811 g 811 g

X = 4.95 gX = 4.95 g