Name ___________________
Period____Date ___
HUMAN RESPIRATION LAB
In this lab we will use a common indicator called phenolphthalein,
which is colorless in acids and turns pink in the presence of a base like
Sodium Hydroxide (NaOH), being used in this experiment. If you put a
straw into a pink solution and blow CO2 into the liquid, the carbon dioxide
will create carbonic acid by reacting with the water and will neutralize the
NaOH. The solution changing from basic to acidic will cause the
phenolphthalein to change from pink to clear. Phenolphthalein is only pink
when exposed to a base, like NaOH. After you blow carbon dioxide into the
flask you will add drops of NaOH to your flask until it changes back to the
same pink as the control flask. You are “titrating” to determine how much
CO2 was expelled by seeing how much NaOH is needed to neutralize it.
The number of drops used will be proportional to how much
carbon dioxide you exhaled into the flask.
Objectives:
 to compare the amount of carbon dioxide produced through
metabolism, during periods of rest and exercise.
 to observe the use of an acid/base indicator
Answer this question, it will be the hypothesis that you refer to in your conclusion:
Situation A: Resting
Situation B: Exercising (did jumping jacks for 3 minutes)
It is hypothesized that a person in situation A or situation B would breathe out more
carbon dioxide? Explain why.
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Methods
Materials:
Graduated cylinder, 100 mL
3, 250 mL flasks
sodium hydroxide (NaOH) solution
phenolphthalein solution
2 straws
pipettes/droppers
stop watch/Timer
Procedure:
1. Measure 100 mL of tap water in a 100 mL graduated cylinder and
transfer it to a 250 mL flask.
2. Add 5 drops of phenolphthalein indicator to this, the control flask,
and set aside. Add 5 drops of NaOH to make it light pink. Set aside
as the comparison/control flask.
3. Repeat steps 1 and 2 for a new flask, the “rest flask.”
4. Taking normal breaths, submerge a soda straw in the solution and
blow through it, into the flask, for exactly one minute. Breathe in
normally (without the tube in mouth) but exhale through
the straw. (Take mouth off straw when you inhale to make sure you
don’t inhale any solution.) Partners should observe the color change
as you blow.
5. This step must quickly follow step 4.
Now slowly and carefully, counting drop by drop, add sodium
hydroxide (NaOH) solution to the solution in the flask using a pipette.
Swirl the water gently in the flask WHILE adding the NaOH.
Continue to add NaOH until the light pink color stays for one minute
and matches the control flask. Record in Table 1, how many drops
were added.
6. Repeat steps 1 and 2 again for an “exercise flask.” (Flask #3)
Now the same person who rested and blew in the flask before will
alter his/her rate of metabolism by doing jumping jacks or some
other appropriate (RIGOROUS exercise) for 3 minutes. Immediately
after, repeat steps 5 and 6. Again record data in Table 1.
7. Convert the number of drops of NaOH into grams of CO2 by
multiplying the number drops by .044 (the conversion factor). Show
work in calculations section and record in Table 2.
Results
Table 1. Drops of NaOH needed to titrate CO2 expelled into water
Situation
Rest
Exercise
# of drops of NaOH
Calculations:
Convert the number of drops of NaOH into grams of CO2 by multiplying
the number drops by .044. SHOW YOUR WORK and keep track of units
(drops or grams).
# of drops of NaOH x .044 = grams of CO2
REST
EXERCISE
Table 2. Amount of CO2 expelled
Situation
Rest
Grams of CO2
Exercise
Analysis:
1. Was there a difference in the amount of CO2 expelled after rest vs.
right after exercise? Use results as clear evidence.
2. Explain your answer to #1 above in terms of cellular respiration and
breathing rate.
3. Clearly explain the role of the phenolphthalein in the experiment.
What kind of indictor is it? Why was it colorless when you exhaled,
but pink when titrated with NaOH?
4. What was the independent variable in this experiment?
dependent variable?
The