CHEMISTRY EXPERIMENT NO. 4 Name__________________________ Class time________
Mole Relations
In this exercise, we will study a reaction involving two common chemicals: hydrochloric acid (HCl (aq),
stomach acid) and calcium hydroxide (Ca(OH)2, a substance chemically similar to Mg(OH)2, an active
ingredient in antacids). An acid-base neutralization reaction occurs when the base, Ca(OH)2 reacts
with HCl(aq) to produce water and a salt, calcium chloride (CaCl2).
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Q1. (a) Write a balanced chemical equation for the acid-base neutralization reaction:
_________________ + ______________  _______________ + _____________
calcium hydroxide
hydrochloric acid
calcium chloride
water
(b) The mole relation specified in this balanced equation is: _____________________________________
To determine experimentally the mole relations between Ca(OH)2 and HCl in the reaction, we will add
hydrochloric acid to calcium hydroxide in appropriate amounts as indicated by a color change of an acidbase indicator: bromothymol blue. When excess base (Ca(OH)2) is present, this indicator shows a blue color.
Its color changes to yellow when the base is completely neutralized by the acid (HCl). By delivering HCl (aq)
slowly through a plastic pipette, we can catch the exact point at which color change occurs. At this point, the
two reactants are mixed in the proper mole relation of the reaction. By weight measurements and gram-tomole conversions, we can then calculate the actual moles of HCl needed to reach this point, and relate that to
the moles of Ca(OH)2 used. The ratio gives us the experimental mole relation of the two reactants.
By comparing experimental mole relation to what is expected from the balanced chemical equation, you will
experience firsthand how a scientific theory (i.e. the mole relation concept) must be backed up by accurate
and reproducible experimental results.
In part B of this exercise, you will determine the percentage of Ca(OH)2 present in a mixture of Ca(OH)2 and
an inert salt, NaCl. This is done by measuring the HCl solution needed to completely react with a known
amount of the mixture, and calculating the weight of Ca(OH)2 present in the sample based on the mole
relation between HCl and Ca(OH)2.
I. Experimental Procedure
Part A.
Determining Mole Relation Between HCl and Ca(OH)2
1. Obtain a plastic storage pipette containing about 20 mL HCl (aq). On a laboratory balance, measure and
record the weight of the pipette in the appropriate blank provided in Table 1 (next page).
2. Obtain a clean, 100 mL beaker. Measure and record its weight in Table 1.
3. With a metal spatula, transfer about 0.120 gram of solid Ca(OH) 2 into the beaker. Measure and record in
Table 1 the exact weight of the beaker with calcium hydroxide. Subtract from the weight of the empty
beaker to find the net weight of calcium hydroxide. Add about 20 mL distilled water and 3 drops of
bromothymol blue indicator to the beaker. Stir with a glass stirring rod or magnetic stirrer to disperse the
solids, which will not completely dissolve. Note the color of the indicator.
4. Deliver HCl(aq) drop-by-drop into the beaker from the storage pipette. Stir with the glass rod or magnetic
stirrer to mix well. Note color changes in the solution. Continue adding more HCl (aq), stir, and observe
color changes until indicator color remains yellow permanently or at least for more than two minutes.
5. Weigh the storage pipette and record its weight in Table 1. Subtract from the original weight of this
pipette to find the net weight of HCl(aq) delivered into the beaker.
Table 1. Data And Calculations
Initial weight of pipette (step 1)
final weight of pipette (step 5)
Formula Weight Ca(OH)2 _____________ g
net weight of HCl(aq) delivered
Formula Weight HCl ____________ g
weight of empty beaker (step 2)
weight of beaker and Ca(OH)2 (step 3)
net weight of Ca(OH)2
Answer the following. Show calculations and pay attention to significant figures. Remember to report the
unit of every numerical value.
Q2. From the net weight of Ca(OH)2, calculate the mole(s) of Ca(OH)2 used in the reaction.
(Net WtFormula Wt)
______________________________________________________________________________________
Q3. Given that 3.7 grams HCl are present in 100 grams of the HCl solution used in this experiment,
calculate the grams of HCl added to the beaker. (Hint: Use the equality “100 g HCl (aq) = 3.7 g HCl” to
convert net weight of HCl(aq) delivered into grams of HCl.)
(grams present in 100 grams * Net wt HCl)
______________________________________________________________________________________
Q4. From the answer to Q3, calculate the mole(s) of HCl delivered into the beaker.
(Q3formula wt acid)
_____________________________________________________________________________________
Q5. (a) Compare the mole(s) of Ca(OH)2 calculated in Q2 to the mole(s) of HCl calculated in Q4. Find the
nearest whole - number - ratio between the two values. (For example, to find the nearest whole -number
- ratio of 0.88 and 0.91, you can divide 0.88 by 0.91= 0.96, which rounds to the nearest whole number:1
therefore, the nearest whole -number - ratio is 1:1.) (Q4Q2)
_______________________________________________________________________________________
b) The ratio calculated in (a) is your experimental mole relation between Ca(OH) 2 and HCl. Is this
experimental mole relation consistent with the mole relation from the balance equation in Q1?
Explain.
___________________________________________________________________________
Part B.
Determining the Percentage of Ca(OH)2 in a Mixture
Put on safety goggles to protect your eyes!
1. Obtain a plastic storage pipette containing about 20 mL HCl (aq). On a laboratory balance, measure and
record the weight of the pipette in the appropriate blank provided in Table 2.
2. Obtain a clean, 100 mL beaker. Measure and record its weight in Table 2.
3. Obtain a sample of Ca(OH)2 mixed with an unknown amount of NaCl(s), which does not react with HCl.
Record the sample code in Table 2. Weigh 0.120 g of your sample into the beaker. Measure and record in
Table 2 the exact weight of the beaker with the mixture. Subtract from the weight of the empty beaker to
find the net weight of the mixture. Add about 20 mL distilled water and 3 drops of bromothymol blue
indicator to the beaker. Stir with a glass stirring rod to disperse the solids, which will not completely
dissolve. Note the color of the indicator.
4. Deliver HCl(aq) drop-by-drop into the beaker from the storage pipette. Stir with the glass rod to mix well.
Note color changes in the solution. Continue adding more HCl(aq), stir, and observe color changes until
indicator color remains yellow permanently or at least for more than two minutes.
5. Weigh the storage pipette and record its weight in Table 2. Subtract from the original weight of this
pipette to find the net weight of HCl(aq) delivered into the beaker.
6. Deduction
0.0
%-Off
2 +/-
Deduction
2.0
%-Off
4 +/-
Deduction
4.0
%-Off
6 +/-
Deduction %-Off
6.0 10 +/- or greater
Table 2. Data And Calculations ( Sample Code:_________ )
Initial weight of HCl pipette (step 1)
final weight of HCl pipette (step 5)
net weight of HCl(aq) delivered
weight of empty beaker (step 2)
weight of beaker and mixture (step 3)
net weight of mixture
Answer the following questions. Show calculations and pay attention to significant figures. Remember to
report the unit of every numerical value.
Q6. Convert net weight of HCl(aq) delivered into grams of HCl based on information given in Q3 on page 2
of this handout.
(grams present in 100 grams * Net wt HCl)
_______________________________________________________________________________________
Q7. From the answer to Q6, calculate the moles of HCl delivered into the beaker.
(Q6formula wt acid)
_______________________________________________________________________________________
Q8. From the mole relation of the neutralization reaction between Ca(OH) 2 and HCl, calculate the moles of
Ca(OH)2 present in your sample of mixture.
(Q7coefficient)
______________________________________________________________________________________
Q9. From the moles of Ca(OH)2 found in Q8, calculate the grams of Ca(OH)2 present in your sample of
mixture.
(Q8*formula wt)
______________________________________________________________________________________
Q10. From the grams of Ca(OH)2 found in Q9, calculate the percentage of Ca(OH)2 present in your
sample of mixture.
(Q9net wt)*100
______________________________________________________________________________________
II.
Practical Applications
Many who suffer from indigestion find relief by taking over-the-counter antacid tablets. This is because
indigestion is often caused by an overproduction of stomach acid (HCl), and HCl can be neutralized through
acid-base reactions. Though there are a variety of over-the-counter antacid formulations, most contain
common mild bases such as sodium bicarbonate (NaHCO3), calcium carbonate (CaCO3), magnesium
carbonate (MgCO3), aluminum hydroxide (Al(OH)3), and magnesium hydroxide (Mg(OH)2).
Q11. Write a balanced chemical equation between each of the above named antacid ingredients and HCl.
Find the mole relation between HCl and each base.
Balanced chemical equation
Mole relation between
HCl and the antacid
_____ HCl + _____ NaHCO3  ____ CO2 + ____ NaCl + _____ H2O
_____ HCl + _____ MgCO3  ____ CO2 + ____ MgCl2 + _____ H2O
_____ HCl + _____ CaCO3  ____ CO2 + ____ CaCl2 + _____ H2O
_____ HCl + _____ Al(OH)3  ____ AlCl3 + _____ H2O
_____ HCl + _____ Ca(OH)2  ____ CaCl2 + _____ H2O
Q12. You are given three different brands of antacid. The first contains 100 mg calcium carbonate per
tablet, second brand contains 60 mg Ca(OH)2, and third contains 75 mg aluminum hydroxide. Which
brand offers the most “neutralizing power” per tablet? Show work and explain. Use the last 3 reactions
listed in the above chart. Calculate either mass or moles, then compare values to answer the question.
The best antacid will neutralize the most acid.

mass of tablet (g)
coefficien t of acid

*
formula
wt
of
tablet
(g/mol)
coefficien
t of tablet


  moles of acid (mol)


mass of tablet (g)
coefficien t of acid

*
* formula wt of acid (g/mol)
 formula wt of tablet (g/mol) coefficien t of tablet
#1) Calculations for 100 mg calcium carbonate
#2) Calculations for 60 mg Ca(OH)2
#3) Calculations for 75 mg aluminum hydroxide

  mass of acid (g)
