What is Stoichiometry?

Definition: the relationship between amount of reactants used in chemical reaction and the amounts of products produced by reaction.

With Stoichiometry students will be able to predict the amount of a specific product that is created when a given amount of reactant is used.

Bolded Coefficients in a reaction equation:
2H2 + 1O2 = 2H20

these coefficients can refer to the amount of  molecules.

ratio:
H2   =  2                                                    2 mol H2
O2       1                 also known as             1 mol O2


eg.

1N2+3H2 --> 2NH3    How many molecules of N2 are required to react with 14 molecules of H2?

15 mol H2 x  1 mol N2
                      3 mol H2     =    5 molecules

Lab 6B

Although we've done this lab before, this time we'll be looking at it in more detail.  Now, we are trying to find the percent yield.  Labs are just a combination of everything that we've already done.  Just remember all the individual steps and watch as the things often talked about happpen before your eyes! 

What is percent yield?

It's the amount of product produced in relation to the amount expected to be produced, expressed as a percent. 

percent yield = actual amount(g)

                       predicted amount(g)

Notes:
In lab 6B, we were given the molarity and volume of the solutions.  This reaction is a double replacement reaction which produces a precipitate.
In order to find the percent yield, we had to make several calculations.

Steps:
1) write a balanced equation for the reaction. 
2) calculate the moles of each solution.
3) find the limiting reactant.
4) use the amount of limiting reactant to calculate the amount of the precipitate expected to be produced. (MUST USE THE AMOUNT OF PRECIPITATE BECAUSE NOT ALL THE REACTANTS WERE USED IN THE REACTION, SO, SOME REACTANTS WILL STILL BE LEFT WITH THE AQUEOUS NaCl.)  This is your predicted amount.

5)  Find the mass of the precipitate actually produced by subtracting the mass of the filter paper from the combined mass of the filter and precipitate.

6) Calculate the % yield using the numbers you just calculated. 

Try this problem. 

If 800. mL of 0.648 M Aluminum Chloride was reacted with 500. mL of 0.5 M hydrochloric acid, what is the percent yield of Aluminum Chloride?

Write a balanced chemical equation, find the number of moles per each reactant, find the limiting reactant and calculate the percent yield.

Excess & Limiting Reactants

Sometimes, in reactions, certain conditions aren't present for a reaction to occur (ex. temperature, pressure etc..)

Because it's not always possible for every atom and molecule to form together during a reaction, we have to add more of one reactant in order to match up the atoms/molecules

This leaves us with:

-one reactant, the one that's used up completely, being the LIMITING REACTANT

-the other reactant, the one in excess is called the EXCESS QUANTITY


Still confused? Use this example to help!

You are selling lemonade at a lemonade stand. You buy 20 lemons and 103 sugar cubes. Each pitcher needs 5 lemons and 22 sugar cubes to make the perfect pitcher of lemonade.

1. How many pitchers can you make?
2. What is the excess ingredient? How much is it in excess?

-Well, since it takes 5 lemons per pitcher, and you have 20 lemons, you can make a total of FOUR    PITCHERS
-Since you're making 4 pitchers, and 22 x 4 = 88, and 103-88= 15, you have a total of 15 ICE CUBES IN EXCESS

Make more sense? :) Let's apply it to some chemistry!

Example:

5.00 g of oxygen gas reacts with 3.00 g of ammonia to produce nitrogen oxide and water. Which reactant is the limiting reactant? How much is in excess?

1) First step, write a balanced equation

                                                           5O₂ + 4NH₃ --> 4NO + 6H₂O

2) Next step, convert the grams of eachs reactant into one of the products using the number of moles in the balanced equation

5.00 g O₂ x  1 mol O₂  x  4 mol NO x  30.0 g NO  = 3.75 g NO
                    32.0 g O₂     5 mol O₂      1 mol NO

3.00 g NH₃ x 1 mol NH₃ x 4 mol NO x 30.0 g NO  =  5.294 g NO
                     17.0 g NH₃   4 mol NH₃  1 mol NO

Since there's more NO produced from the NH₃ than the O₂, we can conclude that the excess quantity is NH₃ and the limiting reactant is O₂.

3) To find how much there is of the excess, take the grams of the limiting quantity and convert it to the excess quantity. This helps us find the actual amount used in the reaction

5.00 g O₂ x 1 mol O₂ x 4 mol NH₃ x 17.0 g NH₃  = 2.125 g NH₃
                   32.0 g O₂    5 mol O₂      1 mol NH₃

Then, subtract original amount from the actual amount used up in the reaction:

                                                      3.00 g      <--original amount
                                                      2.125 g    <--amount actually used
                                                      0.875 g

Therefore, 0.875 g of NH₃ is left in excess

Want some more help? http://www.youtube.com/watch?v=lj-fdEqTBRM&feature=related

Percent Yield and Purity

Usually when calculating for the % yield, you can discover three things; the % of the product actually being used; the amount of the product obtained and the product expected. Basically it is possible to get the whole product through this equation:

% yield = grams of actual product recovered......     x   100%

grams of product expected from stoich     

step 1: MAKE SURE IT'S BALANCED

step 2: find the the amount in grams that are in the element you are trying to find from the given amount of another element or compound (for example: Br in grams from HBrO3). Either way you'll need to convert into moles, then compare moles and then convert back into grams to get the desired answer of the element in grams.

step 3: now simply follow the equation: used the obtained given amount (the obtained Br) and divide it by your results from the expected amount (step 2; in this case, expected Br). finish by multiplying by 100%.

Note: wrtie "% Yield" on your answer. A written answer would sound like " of every 100g there is -so 'n so amount- in grams of the product actually formed."

Say you get a question where they only give you the percent that's used and only grams of a compound.

You still have to find the expected amount from the resulted product (or compound; which is still the example of Br from HBrO3). Next would be simple algebra to find the obtained amount.

Percent Purity

We need to find the % purity in order to calculate how much of the product will form, which is how much is actually able to react. ("the ratio of mass of a pure substance to mass of impure substance expressed in a percent.") % purity has a similar equaiton:

% purity = Mass of pure substance        x   100%

Mass os impure substance