Chemistry PS1 7 Mole/Stoichionetry

Example 1

You are given a mixture of 5 mole H2 gas, 5 mole O2 gas and produce 4 moles H2O

What is the theoretical yield, actual yield and % yield

START BY writing the balanced equation

    2H2   +     1O2   -->  2H2O     Equation coefficients give ratio of moles reacting                                                                                                                                 2 moles    1 mole       2 moles

 

2 moles of hydrogen need 1 mole O2

Each mole of hydrogen needs half that amount of oxygen = 0.5 mol O2

 

To determine the limiting reagent, START with ONE of the amounts given in the question (it doesn't matter which reagent you start with)

 

         —example here: 5 mole H2

5 mole H2  (amount available) need 2.5 mole O2.        

 

But there are 5 mole O2 available (amount given in the question) and you only need 2.5 mol—so oxygen is in EXCESS; 5 mol - 2.5 mol = 2.5 mol O2 in excess

 

H2 is the LIMITING reagent--this is the reagent that is all used up

 

Now to calculate the theoretical yield of the product, water:

Again, look at the coefficients in the equation

      2H2   +     1O2   -->  2H2O  

  2 mole H2 will produce 2 mole H2O

With 5 mole H2, 5 moles H2O are produced, this is the theoretical yield

​

To calculate the percentage yield:

Theoretical yield =  5 moles H2O

actual yield (given in question)  = 4 moles H2O

 

Percentage yield  =    actual yield     x 100   %       = 4 moles x 100                                                   theoretical yield                          5 moles

 

                         % yield = 80%

 

Example #2

% yield example 

To make a toy car: you need 4 wheels, 1 body, 3 stickers

You have enough parts to make 200 cars

                              BUT only 168 are made

 

                The theoretical yield  =  200  Max possible amount that can be produced

                       The actual yield = 168     = Amount that IS produced 

                      the percent yield =  actual yield x 100 %

                                              theoretical yield                                                                                                                                                                                                                     =  168  x 100     =    84%

                                                 200

            84%  = percentage of theoretical (possible) yield that you actually produce                                       (actual yield/theoretical yield) x 100

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Example #3

Converting grams to moles for limiting reagent calculations

32g H2 gas with 96 g O2 to give 99 g water What is the percentage yield

STEP 1:  write equation

STEP 2:  convert O2 and H2  masses to moles                                                         STEP 3: determine the limiting reagent

STEP 4: determine the theoretical yield of water formed

Step 5: determine the percentage yield

​

STEP 1:   write equation

   2H2   +     1O2   -->  2H2O     Equation coefficients give ratio of moles reacting          2 moles    1 mole       2 moles

 2 moles of hydrogen react with1 mole O2

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STEP 2:   convert masses of O2 and H2 given in the equation to moles

moles oxygen  =      mass       =        96       =     3 moles O2

                         molar mass        2(16.00)

moles hydrogen  =      mass       =        32       =     16 moles H2

                           molar mass          2(1.01)

​

STEP 3: determine the limiting reagent

 2H2   +     1O2   -->  2H2O    

 2 moles of hydrogen react with1 mole O2

we have 16 mol H2 which would need 8 mol O2

But we only have 3 moles O2

So, oxygen is the limiting reagent

(and hydrogen is in excess)

       96 g O2 = 3 mole O2 = limiting

        32 g H2 = 16 moles H2  ONLY 6 mol are needed to react so 10 mol are in excess 

​

STEP 4: determine the theoretical yield of water formed

2H2   +     1O2   -->  2H2O  

2 mol H2  react to give  2 mol H2O   with enough oxygen (1 mol)

with the amounts in the question and 3 mol O2 calculated as the limiting reagent

 6 mol H2 react with 3 mol O2 to give 6 mol H2O

What is the mass of water formed?

Mass of 6 mol water:   Mol x molar mass = mass

                               6 mol x [2(1.01) + 16.00] = mass of water

                                                                     = 108 g water

​

STEP 4:  calculate the percentage yield

Theoretical (calculated) yield = 108g

Actual yield (given in question) = 99 g

 

Percentage yield  =    actual yield  x 100   %       =  99 g  x 100 %                                                             theoretical yield                 108 g

 

                                                          % yield = 92%

​

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Example #4

PS 1 7 Percentage composition and moles Chem S Tree

 

A student reacted 1.5 g salicylic acid C7H6O3 with 2 g acetic anhydride C4H6O3  This yielded 1.5 g aspirin C9H8O4

1C7H6O3    +    1C4H6O3 --> 1C9H8O4 + 1HC2H3O7

 1.5 g                2 g     <--- convert to moles  (divide mass given  by molar mass)

0.011 mol      0.020 mol

 

Need molar masses of reactants-- work them out! And divide the masses given by their respective molar masses

Which is limiting reagent? 

Find the number of moles--look at the coefficients in the equation                           1 mole salicylic acid = 1 mole acetic anhydride

So, the smaller number of moles calculated from the masses of reactants is the limiting reagent

Salicylic acid is LIMITING

Acetic anhydride is in EXCESS (0.020 - 0.011 = 0.009 mol acetic anhydride in excess)

 

Now to find how much aspirin is produced?  To find mol aspirin look at the coefficients in the balanced equation.

​

For salicylic acid : acetic anhydride : aspirin

-->           1       :            1          :     1       =  mole ratio,

so  0.011 mol   :        0.011 mol  :   0.011 mol  aspirin formed 

​

The limiting reagent--here 0.011 mole salicylic acid determines (limits) how much can be produced

 

Mass aspirin calc:

        1 mol salicylic acid   : 1 mol aspirin

0.011 mol salicylic acid -->  0.011 mol aspirin       

:for C9H8O4  180 g aspirin (mol mass)     

mass 0.011mol aspirin = mol x molar mass = 1.98 g aspirin

                                                           = theoretical yield

                         
Percentage yield =    actual yield x 100 %

                          threoretical yield                                                                                                                                                                                                                  1.50  x 100     =    76%

                              1.98

 

EXTRA ACTIVITIES

 

Solve crime with this

Who killed Chem S. Tree?

4 suspects, all with a different herbicide residue

see free download of activity at https://www.teacherspayteachers.com/Product/CSI-Empirical-Formula-Who-Done-It-Activity-1065475

or go to https://chemsnyder.wikispaces.com/file/view/CSI+Empirical+Formula+Who+Done+It+Activity+Herbicide.pdf

 

+ dimensional analysis warm up with shapes (free download) at https://www.teacherspayteachers.com/Product/Dimensional-Analysis-Warm-Up-1065433