Lab 6D Stoichiometry

Objectives:
To observe the reaction between solutions of sodium carbonate and calcium chloride
To determine which of the reactant is the limitng and which is the excess reactant
To determine the theoretical mass of precipitate that should form
To compare the actual mass with the theoretical mass of precipitate and calculate the percent yield

Supplies:
centigram
two 25mL graduated cylinder
beacker (250mL)
wash bottle
safety goggle
filtering apparatus
ring stand
250 mL funnel
filter paper
lab apron

Chemicals
25 mL of calcium carbonate
25 mL of sodium chloride

Procedure:
1. Put on lab coat and goggles
2. Get two 25 mL graduated cylinder and a 250 mL beaker
3. Measure 25 mL of calcium carbonate and sodim chloride and pour them each into the 25mL graduated cylinder
4. Combine the two chemicals by pouring them both into the 250 mL beaker. Wait for 5 minutes while observing what happens
5. Write your names on the filter paper and then weigh the filter paper. Set up the filtering apparatus with it
6. Use wash bottle to wet the bottom of the filter paper to keep it in place
7. Slowly stir the newly mixed chemical into the filter as it filters the liquid into the beaker. This will take a while so be patient.
8. Once all filtered, take the filter paper our and place it on a paper towel to dry.
9. Remember to clean up and wash your hands once you are done on day one of the experiment.
Day 2:
Weigh the precipitate left over from day 1.

Follow-Up

We measured the amount of CaCO3 produced by our experiment in grams with a centigram. Then, after balancing the equation "1Na2CO3 + 1CaCl2 ---> 2NaCl + 1CaCO3" we calculated the percent yield for our experiment.

The theoretical yield for CaCO3 is 1.15125 grams = 1.3grams.
The actual yield for our experiment was 1.18 grams.
So the percent yield is (1.18/1.125125) x 100% = 94.3%

Excess and Limiting Reactants Percent Yield

Excess Quantity
A balanced equation shows what should happen in a chemical reaction, but it does not always happen due to specific conditions such as pressure, temperature, and/or concentration.

In some reactions, one reactant will be left over, it is called the excess quantity. The second reactant is used up completely, and it's the limiting reactant(lower value).

Ex/ What mass of OCl2(g) is produced when 98.0 g of O2(g) and 134.0 g of Cl2(g) are reacted?

First we need a balanced equation for this reaction.

O2 + 2Cl2 -> 2OCl2

We take the two values of O2 and Cl2, and convert them both into grams of OCl2

98.0 g O2 x 1 mole O2 x 2 moles OCl2 x   87 g OCl2  = 533g OCl2
                    32 g O2       1 mole O2         1 mole OCl2

134.0g Cl2 x 1 mole Cl2 x 2 moles OCl2 x   87g OCl2   = 164g OCl2
                      71 g O2       2 moles Cl2       1 mole OCl2

Now we know that 164g is the limiting reactant, and that is the answer.
If we are asked how much of which reactant will be left over, we take the limiting reactant's value and convert it to the excess quantity.

134.0g Cl2 x 1 mole Cl2 x 1 mole O2  x   32 g O2  = 30.2 g O2
                     71 g Cl2       2 moles Cl2   1 mole O2

Finally, we take the original value and subtract it by the new value and get the amount that's in excess.

98.0 g O2 - 30.2 g O2 = 67.8 g O2 in excess

This video gives a different example and offers a good explanation as well.

(continued)

In a chemical equation, the reactants are not used up all the time, and the products rarely produce what they are predicted. We use percent yield to calculate the amount of products obtained in an equation.

Formula: Percent yield =   grams of actual product recovered             x 100 %
                                  grams of product expected from stoichiometry

Example: 2 C3H8 + 7 O2 --> 6 CO + 8 H2O

          If 100.0 g of C3H8 is reacted with excess O2 and 50.0 g of CO was produced, what is the percent   
         yield ? 


Step 1: convert C3H8 into CO

         100.0 g C3H8 x 2 mol C3H8 x 6 mol CO   x  18 g        = 40.9 g CO
                                    44 g C3H8    2 mol C3H8  6 mol CO



Step 2 : Use the formula.


                  Percent yield =      50.0 g CO recovered  x 100% = 122 % 
                                               40.9 g CO obtained

 Sometimes not all reactant is available to react. We calculate how much there are by using percent purity and here's the formula for it.  mass of pure substance  x 100 %

                                            mass of impure sample