Experiment-12: Analysis of an Antacid
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Back Titration
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Molarity calculation
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Antacid active ingredients
The human digestive system uses hydrochloric acid to help breakdown food in the
stomach. The parietal cells in the stomach secrete hydrochloric acid at a concentration of
about 0.155 M HCl (pH 1-2), quite concentrated! The flow of HCl increases when food
enters the stomach. When you eat or drink too much, your digestive system may generate
too much acid. You may develop a condition called "heartburn" or indigestion. Antacids
are ingested to reduce indigestion caused by excess stomach acid. Antacids neutralize the
excess acid and "relieve" but not eliminate the condition. The reaction that takes place is
an acid/base reaction. A little bit of NaOH might be equally effective, but it is extremely
rough on the rest of the digestive system, so antacids need to be formulated to reduce
acidity while avoiding physiological side-effects. Therefore, active ingredients of
commercial antacids are weak alkaline (basic) chemicals. In addition to the active
ingredient (base), Antacids also contain flavors, sweeteners, binders, fillers, antifoam
agents, pain relievers (aspirin), etc. The base in antacids varies with the brand as shown
below:
Brand Name
Active Basic Ingredient(s)
Pepto-Bismol
BiO(HOC6H4COO)
Milk of Magnesia Mg(OH)2
Rolaids
NaAl(OH)2CO3 (newer tablets: CaCO3)
Tums
CaCO3
Alka-Seltzer II
NaHCO3 and KHCO3
Maalox
Mg(OH)2 and Al(OH)3
Gaviscon
Al(OH)3
The active basic ingredients will neutralize excess acid as per the following equations:
BiO(HOC6H4COO) + 3 H+(aq) Bi3+(aq) + H2O(l) + HOC6H4COOH(s)
Mg(OH)2(s) + 2 H+(aq)  Mg2+ (aq) + 2 H2O(l)
Al(OH)3(s) + H+(aq)  Al(OH)2+ (aq) + H2O(l)
CaCO3(s) + H+(aq) Ca2+(aq) + HCO3- (aq)
HCO3- (aq) + H+(aq) CO2(g) + H2O(l)
In this experiment, you will analyze Milk of Magnesia, which contains magnesium
hydroxide (Mg(OH)2 ) as the active portion. Your goal is use volumetric analysis to find
out the amount of Mg(OH)2 present in 5.00 mL Milk of Magnesia. The side panel on the
bottle indicates that each 5.00mL portion of Milk of Magnesia delivers 400 mg of
Mg(OH)2 . Therefore, the percent error can be calculated at the end of your experiment.
The excess acid in the stomach will be neutralized by Milk of Magnesia as per the
balanced reaction: Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l)
The technique you will use today is known as Back Titration. First, a 5.00 mL portion of
Milk of Magnesia will be neutralized using more than enough volume (16.0 mL) of a
standard HCl(aq) (1.00M). Then, any remaining excess of HCl(aq)will be titrated against a
standard NaOH(aq) solution as in the last experiment. You can calculate the mol NaOH of
used in the titration, since the molarity (0.1M) and volume of NaOH(aq) is known.
Because NaOH and HCl react in 1:1 molar ration, the mol NaOH is equal to the
remaining excess of mol HCl.
[Mols of standard HCl used] = mols of HCl neutralized by Milk of Magnesia +
remaining excess of mol HCl
[Molarity x Volume of standard HCl] = X + (mol NaOH)
[1.0 M x 16.0 x 10-3 L]
= X + (Molarity x Volume in L of standard NaOH)
[16.0 x 10-3 mol]
= X + (0.1M x Volume of NaOH used ÷ 1000)
X = [16.0 x 10-3 mol] - (0.1M x Volume of NaOH used ÷ 1000)
Mols of Mg(OH)2(s) per 5.00 mL of Milk of Magnesia = 0.5 x X mol
Convert 0.5 x X mol of Mg(OH)2 into grams = 0.5 x X x molar mass of Mg(OH)2
Procedure:
1. Sample Preparation: Shake the Milk of Magnesia bottle well before dispensing.
Using a small (10-mL capacity) DRY graduated cylinder, transfer exact 5.0 mL
Milk of Magnesia into a clean Erlenmeyer flask. If inside of this Erlenmeyer flask
is wet, it will not affect your results. To remove all the Milk of Magnesia, which
is stuck to inside wall of small cylinder, rinse it with 10 mL distilled water and
transfer into the Erlenmeyer flask. Rinse two more times with 10 mL distilled
water (total water added is about 30 mL).
Carefully swirl the contents of
Erlenmeyer flask for 1 minute.
2.
Using a DRY graduated cylinder, carefully obtain exact 16.0 mL of 1.0 M HCl
(CAUTION: HCl is CORROSIVE!) from the big supply container. Do not
remove the big supply container. Slowly transfer this HCl into the Erlenmeyer
flask prepared in step-1. Carefully swirl the contents of Erlenmeyer flask for 2-3
minutes. Add 3 drops of phenolphthalein. Swirl the flask for 1 more minute to
mix the contents thoroughly. At this stage, there are four items in your
Erlenmeyer flask (exact 5.0 mL Milk of Magnesia + about 30 mL distilled water
+ exact 16.0 mL of 1.0 M HCl +3 drops of phenolphthalein).
Note: A pink color indicates that more HCl is required. Consult your teacher!
3. Buret Set-Up: Review the figure. Transfer approximately 100 mL 0.1 M NaOH
into a 250 mL beaker and take it to your bench. You will use this NaOH to fill-up
the buret. Do not take the big bottle of NaOH to your desk. Wash and rinse a
buret with distilled water. Then rinse this buret twice with 5 mL portions of the
0.1 M NaOH, and drain into the designated waste container. Fill the buret almost
to the top with 0.1 M NaOH. Clamp this buret to the iron stand. Open the
stopcock/control knob slowly to drain out NaOH until the lowest level of
meniscus is at 0.00 mL mark. Make sure there are no air bubbles. 0.00 mL is your
initial reading.
4. Titration: Place the Erlenmeyer flask from step-2 under the buret and start
titration as demonstrated by your teacher. Remember to keep swirling the flask
while adding NaOH in a drop wise and slow manner. One may place a white
piece of paper under the flask to observe the end point (colorless to permanent
lightest pink). Once you reach the end point add no more NaOH. Read the buret.
This is your final buret reading. The difference between final and initial reading is
the volume of 0.1 M NaOH used in this trail#1 (__________mL)
5. Discard the contents of Erlenmeyer flask into the designated waste container.
6. Using a small funnel carefully refill the buret to 0.00 with 0.1 M NaOH solution.
7. Second Trail: Repeat step-1 and 2.
8. Repeat step-4. Volume of 0.1 M NaOH used in this trail#2 (__________mL)
9. One by one discard the contents of Erlenmeyer flask, buret and left over solutions
into the designated waste container.
10. Calculate the molar mass of Mg(OH)2 using a periodic table.
Laboratory Report#12: Analysis of an Antacid
Last Name_____________________________, first name________________
Date of Experiment___________ Instructor’s Initials_______________
Data and Calculations
1. Volume of Milk of Magnesia used in each trail
= 5.00 mL
2. Molarity of NaOH used in each trail
= 0.1 M
3. Volume of NaOH used in trail#1 (Final – Initial buret reading) = ________ mL
4. Volume of NaOH used in trail#2 (Final – Initial buret reading) = ________ mL
5. Average Volume of NaOH used = (#3 + #4) / 2 = ______ mL ÷ 1000 = ______ L
6. Mols of NaOH used = Molarity on NaOH x Volume of NaOH in Liter = step-2 x
step-5 = _____________ mol NaOH = remaining excess mol of HCl
7. Volume of standard HCl used in each trail = 16.0 mL = (16.0 ÷ 1000) L= 0.016 L
8. Exact Molarity of standard HCl used in each trail = 1.0 M
9. [Mols of standard HCl used] in step-2 = step-7 x step-8 = 0.016 mol HCl
10. mols of HCl neutralized by Milk of Magnesia = step-9 – step-6 = _____mol
11. Mols of Mg(OH)2 per 5.00 mL of Milk of Magnesia = (0.5 x step-10) mol=
_____________mol
12. grams of Mg(OH)2 per 5.00 mL of Milk of Magnesia = step-11 x molar mass of
Mg(OH)2
= step-12 x 58.32 g/mol = ________________g
13. milligrams of Mg(OH)2 per 5.00 mL of Milk of Magnesia = step-12 x 1000 =
______________________ mg
14. Actual milligrams of Mg(OH)2 per 5.00 mL of Milk of Magnesia from the side
label = ______________________ mg
15. % error
= [(Experimental Value – Actual Value) ÷ Actual Value] x 100 %
= [(step-13 – step-14) ÷ step-14] x 100 % = +______________ %
Show all your work! Use proper units and significant figures!!
1. Write the chemical names of the following bases:
Mg(OH)2 _________________________________________________
CaCO3
_________________________________________________
NaHCO3 _________________________________________________
KHCO3
_________________________________________________
Al(OH)3
_________________________________________________
2. Calculate the mL of 1.0 M HCl required to neutralize 800 mg of Mg(OH)2 .
1 Mg(OH)2(s) + 2 HCl(aq)  MgCl2(aq) + 2H2O(l)
a. Convert 800 mg of Mg(OH)2 into mols_______________________
b.
Mols of HCl required = 2 x Mg(OH)2 mols ____________________
c. Volume, in Liter, of HCl required
= Mols of HCl required/ Molarity
= __________ L (Note: Molarity is given = 1.0 M)
= __________ mL
3. Why is it necessary to shake the Milk of Magnesia bottle well before dispensing?