What is the Compound in “Mr. Coffee” Cleaner?1
A. Introduction
HOW “MR. COFFEE CLEANER” WORKS
“Mr. Coffee” sells a cleaner that is supposed to clean the ‘scale’ that forms in coffee pots and tea
kettles when water is boiled in them. The substance in this product is an acid and your task is to
determine the molar mass of the acid and then attempt to identify the acid.
You have probably seen scale inside coffee pots, teakettles, on hot water tanks, around the holes
of a steam iron, etc. It’s that rather gray (dirty white) solid that you see on surfaces that have
been exposed to hot water.
‘Scale’ of the type that we are talking about here forms when the water supply used for heating
contains both Ca2+ (Mg2+ behaves in the same manner) and bicarbonate ions (HCO3-). Under the
influence of heat, bicarbonate breaks down as follows:
2 HCO3- (aq)
H2O (l) + CO2 (g) + CO32- (aq)
Then, the calcium cation reacts with the carbonate anion as follows:
Ca2+ (aq) + CO32- (aq)
CaCO3 (s)
In other words, the reaction forms an insoluble solid (calcium carbonate) that precipitates out of
solution. The precipitate moves to the bottom of the heating container by gravity and eventually
becomes baked on to the heating surface. The off-white solid that you see associated with
heating surfaces is frequently calcium carbonate. There are other scales that are formed on hot
surfaces, but we will not deal with them in this experiment.
If you have ever worked with this scale, you know that you can chip it off with a knife or other
sharp-bladed instrument. But there is an easier way. Perhaps surprisingly, you can remove this
type of scale with the addition of acid:
CaCO3 (s) + 2 H+ (from an acid)
Ca2+ (aq) + CO2 (g) + H2O (l)
The scale dissolves, the calcium ion goes back into solution and the carbon dioxide escapes as a
gas.
Now you know why you can remove this form of scale by boiling or soaking the container in
vinegar. Vinegar contains acetic acid (CH3COOH, about 5% by weight) which supplies the H+
ions in the above equation. Other scales, such as those made from calcium sulfate, CaSO4, will
NOT dissolve in this way.
Mr. Coffee contains an acid, but it is not acetic acid. Your task is to react the powdery material
that is in “Mr. Coffee Cleaner” with sodium hydroxide. Based on these results, you are to
determine the molar mass of this acid and then with the aid of additional information, see if you
can identify the acid.
ACID-BASE TITRATIONS
You are going to determine the amount of acid in this cleaner by a technique known as titration.
Mr. Coffee Acid, p. 2 of 5
For this titration you will add a strong base from a buret to neutralize all of the acid.
A typical neutralization process is the reaction between the strong base sodium hydroxide
(NaOH) and the strong acid hydrochloric acid (HC1). As usually performed in the laboratory,
the base would be added from a buret to the acid solution until all of the acid had been
neutralized. As the balanced equation shows, the reaction forms a salt and water.
NaOH + HCl
NaCl + H2O
Please note that 1 mole of base neutralizes 1 mole of acid. Let us assume two points about this
cleaner. In the first place, assume that the acid is monoprotic. In other words, there is only one
acidic hydrogen atom in each acid molecule. In the second place, assume that this cleaner has
only the pure acid in it. That is, there are no inactive or inert ingredients present in the cleaner.
If we designate the acid in Mr. Coffee as HA, then the balanced neutralization reaction would be:
NaOH + HA
NaA + H2O
Once you know the volume of base that has been added and the concentration (molarity) of that
base, you will be able to determine the number of moles of base (recall that M = moles/volume
[in liters]) that have been added to neutralize the acid. Then, use the stoichiometry of the
balanced neutralization equation to determine the number of moles of acid.
Now, combine the number of moles of acid that you calculated from the stoichiometry of the
reaction with the weight of acid that you weighed in the beginning, and you should be able to
calculate the molar mass of the acid (recall that moles = grams/molar mass).
Experimental procedure
PREPARING YOUR BURET
1. Take a clean, dry 200-mL beaker to the stockroom to obtain your NaOH solution. Record
the molarity of the solution. Fill your buret following the following guidelines.
The standard laboratory buret is constructed to deliver 0-50 mL of liquid with a reliability of
± 0.2%. or ±.02mL. Careful adherence to buret techniques is required for obtaining accurate
results.
SAFETY NOTE: Eye protection is essential when handling hydroxide solutions. When
adding a solution to the buret, the mouth of the buret must be below face level to prevent
possible splashing onto your face. Either remove the buret from the clamp or place the entire
ring stand in a lower position.
2. Rinse the buret with 2 or 3 small portions (5 -10 mL) of the solution. Rinsing is important
since water remaining in the buret will dilute a standard solution. Fill the buret to a level
about one-half inch above the zero mark. (Use a funnel or a beaker in this process.)
3. Just prior to beginning the titration, allow the liquid level to fall to the zero mark (drain the
fluid into a waste beaker. The bottom of the liquid meniscus should be used as the point of
Mr. Coffee Acid, p. 3 of 5
reference when reading the buret. Avoid parallax error by keeping your eyes level with the
mark being read. Touch off any drop clinging to the tip.
The following rules apply to all titrations and are included to assist you in your titration.
Use an Erlenmeyer flask for all titrations. Position the tip of the buret about 1 inch down
inside the neck of the flask. Place piece of white paper under the flask.
Control the stopcock with your left hand and continuously swirl the solution in the flask
with your right hand. (Left-handers reverse.) Add the titrant (NaOH) continuously,
slowing the flow rate as the equivalence point is reached. Repeatedly stopping the flow
and shaking the flask wastes time. Near the equivalence point, add drops singly.
Fractional drops may be added by washing them off with a stream from the wash bottle.
(Be sure your wash bottle contains distilled water.) Rinse down the walls of the flask.
TITRATION OF “MR. COFFEE CLEANER”
4. Weigh a sample of the “Mr. Coffee Cleaner” that is between 0.500 and 0.600 grams into a
125-mL Erlenmeyer flask. To do this, use the following procedure. Remember to record all
weighings in your notebook.
1) Fold a piece of weighing paper into a weighing boat as demonstrated by your instructor.
2) Take your "boat" to the stockroom and obtain a sample of “Mr. Coffee Cleaner.”
3) Carefully take your boat with the acid to the balance room and find its mass. Pour the
acid into your 125-mL Erlenmeyer flask.
4) Reweigh the paper and record this value. The mass of your sample is the difference
between these weights.
Be sure to weigh to at least 3 decimal places!
5. Add about 25 mL of distilled water and 3 drops of phenolphthalein indicator to this solution.
Record the initial buret reading in your notebook. Begin titrating with the NaOH solution.
The molarity of this solution will be very close to 0.500 M; you should have been told the
exact molarity when you got your solution; record this value in your notebook.
6. As you add the base to this solution you will begin to see the formation of a pink color just at
the point where the drop of base meets the acid solution. As you swirl the flask this pink
color will disappear. As more base is added it will take more swirling and more time for the
pink color to disappear. When all of the acid has been neutralized by the base the pink color
will remain. "Sneak" up on this point by S-L-O-W additions of base. The entire solution
should turn pink, but this pink should only last for about 30 seconds and after that the
solution should turn colorless. If the solution turns ‘hot pink’ you’ve gone too far, you’ve
added too much base. If you have followed these directions correctly, you will have used
between 12-20 mL of the NaOH to titrate your sample. Record the final buret volume reading
and determine the volume of NaOH delivered.
When the equivalence point is reached, record the reading and double-check it. Always
record ±2 decimal places for each volume reading. With practice, the reading can
estimated to ± 0.02 mL. For example write 24.50 mL, rather than 24.5 mL, since the buret
Mr. Coffee Acid, p. 4 of 5
is capable of more than ±0.1 mL accuracy.
7. Refill the buret with NaOH, empty and rinse the Erlenmeyer flask and weigh a new sample.
Make sure all your data is entered in your notebook. Follow this procedure until you have
done at least 2 good trials (no hot pink). Then do the calculations.
When finished with calculations, rinse the buret with distilled water and return it to the
stockroom.
Chemical Waste
Faintly pink titration solutions are neutral and can be poured down the drain. Excess titration
(NaOH) should be poured into the waste container provided on the center island in the lab.
Calculations
Do the following calculations in your notebook.
1. From the volume of base (Vb) added and the molarity (concentration) of the base (Mb),
calculate the number of moles of base (nb) used.
 1000 mL 

nb (mol ) = M b (mol L ) × Vb (L ) × 
 1L 
2. Use the stoichiometry of the balanced equation to
find the number of moles of acid (na) that reacted
with the base (see page 2).
3. Calculate the molar mass of the acid (MMa) from
Acid
Ascorbic
Citric
Malic
Oxalic
Phosphoric
Salicylic
Succinic
Sulfamic
m (g )
na (mol )
4. Identify the acid in “Mr. Coffee Cleaner” from the
table.
Your report will consist of purpose, conclusion,
data, calculations, and answers to the questions on the next page.
MM a
(g mol ) =
Molar Mass (g/mol)
176.1
192.1
134.1
126.1
82.0
138.1
118.1
97.1
Questions
1. If you had started the titration from 1.00 mL on the buret instead of zero, but forgot that you
did so, how would that mistake affect your results?
2. Why is the exact amount of water that you add to the Mr. Coffee cleaner in the Erlenmeyer
flask not important?
3. If the acid in Mr. Coffee cleaner were diprotic (contains 2 acidic hydrogen atoms per
molecule) what answer would you have obtained for your molar mass? Hint: Use the
following balanced equation. H2A + 2 NaOH Na2A + 2H2O
Mr. Coffee Acid, p. 5 of 5
Prelab:
To test your understanding of the calculations, work the following problem before continuing
with the experiment.
A pure sample of a monoprotic acid was titrated with 0.178 M NaOH solution. If 0.617 grams
of the acid required 18.35 mL of the base for complete neutralization, what was the molar
mass of the acid? In Experiment 1, the buret had an average correction value of +0.04 mL.
1.
What is the corrected volume of base?
2.
Answer
Use the formula in the Calculations section to find the moles of base.
nb
3.
How many moles of acid were in the sample (na)?
na
4.
Calculate the molar mass of the acid (MMa).
MMa
5.
Identify the acid in “Mr. Coffee Cleaner” from the table.
Chemical