Laboratorio Estequiometría

Stoichiometry Lab
In class, you’ve learned to compute how much of a chemical product you can
make when you mix measured amounts of chemical reactants. In this lab, you
will be actually using this information to predict how much product will be made;
you will then calculate the percent yield gained from the amount that you actually
recover.
The reaction you will be working with should befamiliar to you from elementary
school science fair volcanoes: You will be mixing baking soda (NaHCO3) with
vinegar (CH3COOH) to generate carbonic acid (H2CO3, which breaks up into
water and carbon dioxide gas) and sodium acetate (NaCH3COO), which is a food
preservation additive.
Prelab:
1)
If we mix 25 grams of sodium bromide with a large amount of potassium
chloride, what will ourtheoretical yield of sodium chloride be?

2)

If our actual yield from this reaction was 18 grams of sodium chloride,
what would our percent yield for this reaction be?

3)

Is the answer in question 2 reasonable? If so, explain why you think this
was a reasonable answer. If not, explain what is wrong with it and discuss
possible reasons you might get this answer in the laboratory.

4)

Whatare some factors that might cause our percent yield to be greater
than 100%? What are some factors that might cause it to be less than
100%? Make sure you discuss specific cases of how both might happen.

© 1999 Cavalcade Publishing

For chemistry help, visit www.chemfiesta.com

Section 1: Computing the amounts of reactants that we need
In this lab, you will need to do a reaction wherebaking soda will react with an
excess of vinegar. By doing this, you will (hopefully!) ensure that you will get
100% actual yield for the reaction.
For our reaction, we will need to use 0.05 moles of baking soda, which we will
call by its chemical name, sodium hydrogen carbonate, for the rest of this lab. If
we use much more than 0.05 moles of baking soda, the reaction will be too large
andwe will risk having some of the reaction products pour over the side of the
flask when we mix it with the vinegar (which we will call acetic acid).
In the space below, calculate how much sodium hydrogen carbonate we will
need for this lab:

For this lab, we will use _________ grams of sodium hydrogen carbonate.
Section 2: The reaction
In this section of the lab, we will be doing thereaction discussed in the prelab.
1)

Measure out the weight of sodium hydrogen carbonate that you calculated
you would need in the first section of this lab. Make sure that the amount
you use is as close as you can make it to this amount. Write the exact
amount of sodium hydrogen carbonate that you used here:
Amount of sodium hydrogen carbonate used: ______________

2)

Dissolve the sodiumhydrogen carbonate in about thirty milliliters of water.
Stir the solution until most or all of it is dissolved (if a little won’t dissolve,
that’s OK).

3)

Weigh a 500 mL flask. You will need the weight of the empty flask at the
end of the lab.
Weight of the empty 500 mL flask: ______________

4)

Add the sodium hydrogen carbonate solution to the preweighed 500 mL
flask.

© 1999Cavalcade Publishing

For chemistry help, visit www.chemfiesta.com

5)

Obtain 150 mL of acetic acid (a.k.a. vinegar) and slowly add it to the
sodium hydrogen carbonate solution. You will observe the formation of
bubbles when the acetic acid is added to the sodium hydrogen carbonate
solution. Wait until the bubbling subsides before adding more acetic acid.
When all of the acetic acid hasbeen added, stir for two minutes before
moving on to step 6.

6)

When the solution is again calm (there may be a few bubbles rising from
the bottom of the flask - this is normal), move the flask to a hot plate and
heat it to boiling. Be careful that the flask does not boil over because this
will cause errors in your calculations - a good way to prevent this is to add
a boiling stick....