Title: Determining the Efficiency of a Reaction
Purpose: Use grams of NaHCO3 and a balanced chemical equation to predict,
then verify the grams of NaCl produced
Brief Overview:
You will react sodium bicarbonate with an excess of hydrochloric acid.
NaHCO3 (s) + HCl (aq) ---> NaCl (aq) + CO2 (g) + H2O (l)
A careful measurement of the mass of NaHCO3 and knowledge of the balanced
equation (above) will allow you to calculate the mass of NaCl expected to be
produced. Then, you will measure the actual amount of NaCl you produced and
compare it to the predicted amount.
Safety:
1) Make sure to wear goggles and during the entire course of the lab.
2) This lab involves handling 3 M hydrochloric acid. This substance is caustic and
will destroy body tissue, including the eyes. Be careful to avoid spills, splatter or
inhalation of vapor. You may with to wear an apron.
3) If you spill any HCl solution, inform your teacher immediately.
Procedure:
1) Create a data chart to record all your masses.
2) Make sure the electronic balance digital readout shows 0.00. If it shows any
other value, press the "tare" button once (do not hammer at it with your thumb!!).
3) Place an empty 250-mL beaker on the electronic balance. Allow the value to
become steady, which will take about 3 seconds. Record this value in your data
table.
4) Remove the beaker from the balance and add one spoonful (moderately
heaping) of sodium bicarbonate.
5) Return the beaker to the electronic balance and allow the value to become
steady. Record this in mass in your data chart. Now determine the mass of the
sodium bicarbonate and record this in your data chart.
6) Pour about 20 mL of 3-molar hydrochloric acid into a 100-mL beaker. Place a
medicine dropper in the beaker. DO NOT put the medicine dropper down
anywhere but the beaker.
7) Add 3 drops of acid to the NaHCO3 beaker, moving the dropper so that no
drops land on each other. The key point is to spread out the adding of acid so as
to to hold all splatter within the walls of the beaker.
8) Continue to add acid slowly drop by drop. As liquid begins to build up, gently
swirl the beaker. This is done to make sure unreacted acid reaches unreacted
sodium bicarbonate. Do not add acid while swirling.
9) Your goal is to stop adding acid when all bubbling has ceased. Another way to
say that is to add the minimum amount of HCl that will react with all of the sodium
bicarbonate. When you think all bubbling has ceased, swirl the beaker and check
for more bubbling. When you think all bubbling has ceased, add ONE DROP
more of acid and swirl. If no bubbling happens, you are done. DO NOT add more
acid.
10) Evaporate off the liquid by using either a hot plate or a Bunsen burner.
BECAREFUL NOT TO BURN YOUR PRODUCT!! While your solution is
evaporating follow the clean-up steps.
Clean-up Steps:
11) Pour any remaining HCl solution down the drain with the water running.
Thoroughly rinse the 100-mL beaker with tap water. Rinse your pipet thoroughly
in tap water. Dry all glassware and place in appropriate storage. If you handled
the acid, then wash your hands.
12) Once your product is dry allow the beaker to cool, weigh your beaker and
contents on the electronic balance. Record this weight (first weighing) in your
data table.
13) Using a Bunsen burner or hot plate, heat the beaker and its contents again
for 3 minutes. Allow the beaker to cool.
14) Weigh the beaker and contents (second weighing). Record in data table.
15) Repeat the previous two steps (third weighing). Record in data table. As long
as your last to weights are the same you can discontinue heating. Now clean up.
Conclusion Questions:
1) Write the grams of NaHCO3 you had in your beaker.
2) Calculate how many moles of NaHCO3 the mass is.
3) Write the molar ratio for NaHCO3 : NaCl.
4) Write the number of moles of NaCl you predict were produced in your
experiment.
5) Calculate the mass of NaCl you predict will be produced.
4) Determine, by subtraction, the actual mass of NaCl produced in your
experiment.
a) first weighing
b) second weighin
c) third weighing
5) Calculate your percent yield.
6) What are some plausible reasons why your percent yield is less than 100%?