Mass % NaHCO3 in Alka Seltzer
In addition to this presentation, before coming to lab or
attempting the prelab quiz you must also:
 Review the video about use of the balances if needed
 Read the introduction to the lab exercise in the
coursepack
What’s the point?
• Experimentally apply reaction and formula
stoichiometry
• Introduce the idea of limiting reagents
• Chapter 3, Brown, LeMay and Burstein
• Review the concept and use of mass %
• Practice interpretation of graphs
Background
Alka-Seltzer
• medicine used to neutralize stomach acid
by reaction with a base
• the basic component of Alka-Seltzer is
NaHCO3 (sodium bicarbonate)
In the presence of acid (H+), the reaction is
NaHCO3(aq) + HCl(aq)  H2O(l) + CO2(g) + NaCl(aq)
• Notice that a gas (CO2) is generated. This makes the “fizz”
Reaction Stoichiometry
NaHCO3(aq) + HCl(aq)  H2O(l) + CO2(g) + NaCl(aq)
• For each mole of CO2 (g) produced, 1 mole of
NaHCO3 reacts
• We say there is a 1:1 stoichiometric ratio between
CO2 and NaHCO3
• Notice also that for each mole of CO2 (g) produced, 1
mole of HCl must also have reacted
• There is a 1:1 stoichiometric ratio between CO2 and
HCl
Example Calculation: Mass %
• mass percent NaHCO3 in a tablet:
% NaHCO3 = (mass of NaHCO3 / tablet mass) * 100%
An Alka-Seltzer tablet was 3.234 g. Reaction of
the tablet yielded 0.672 g of CO2. What was
the mass % NaHCO3 in the tablet?
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Alka-Selter tablet mass = 3.234 g
CO2 mass = 0.627 g
Hmm, we need the NaHCO3 mass!
How is the CO2 mass related to NaHCO3?
• To relate amounts of two chemicals in a reaction we
use the stoichiometric ratio (i.e., the mole ratio in the
chemical equation).
• We can relate the moles of CO2 to moles of NaHCO3
• To get from g CO2 to moles CO2, use the molar mass.
0.672 g x 1 mole CO2 = 0.0153 mole CO2
44.01 g CO2
• CO2 moles = 0.0153 moles
• Use stoichiometry to convert mol CO2 to mol NaHCO3:
NaHCO3(aq) + HCl(aq)  H2O(l) + CO2(g) + NaCl(aq)
• For each mole of CO2 (g), 1 mole of NaHCO3 reacts
Convert moles CO2 into moles NaHCO3 by stoichiometry
0.0153 mol CO2 x 1 mol NaHCO3 = 0.0153
1 mol CO2
mol NaHCO3
• NaHCO3 moles = 0.0153 moles
• To convert moles NaHCO3 to g, use the molar mass.
0.0153 mole NaHCO3 x 84.01 g NaHCO3 = 1.28 g
1 mole NaHCO3
• Given the mass of NaHCO3 (1.28 g) and the tablet
mass (3.234 g), we can find the mass % NaHCO3:
1.28 g x 100%
3.234 g
= 39.6%
Note: 100% is considered an exact number
Preparing Solutions by Dilution
• In this experiment, you must prepare 1M HCl by
diluting stock 6 M HCl
• Remember, M = molar concentration (mol per L)
The Dilution Equation
M1V1 = M2V2
M1 = initial concentration (before dilution)
M2 = final concentration (after dilution)
V1 = volume of undiluted sample
V2 = volume of diluted sample
Dilution Calculation and Procedure
How many mL of 6 M HCl are needed to
prepare 250 mL of 1 M HCl by dilution?
We know our starting concentration, but
not how much we need to use to get the
final desired volume and concentration,
so…
M1 = 6 M
M2 = 1 M
250 mL
V1 = ?
V2 =
Dilution Calculation and Procedure
M1V1 = M2V2
(6 mol L-1)(? L) = (1 mol L-1)(0.250 L)
V1 = 0.0417 L
So, if we dilute 41.7 mL of 6 M HCl to 250 mL
with DI water, we will have prepared 1 M HCl.
Procedure
• measure 41.7 mL of 6 M HCl into a graduated cylinder
• transfer to a 250 mL volumetric flask
• dilute to the mark with DI water and mix thoroughly
Limiting Reagent
NaHCO3(aq) + HCl(aq)  H2O(l) + CO2(g) + NaCl(aq)
• If you had 1 mol of NaHCO3 but only 0.5 mol of HCl,
you’d run out of HCl before all the NaHCO3 reacted.
• Then, HCl would be the “limiting reagent” (LR): the
amount of CO2 produced would be limited by the
amount of HCl.
• The moles of product obtained is is based on the
moles of LR:
0.5 mol HCl x 1 mole CO2 = 0.5 mole CO2
1 mole HCl
Safety
• lab goggles and coats must be on
• use care in diluting the desktop 6M HCl
to 1M HCl
• HCl is a strong acid and is caustic
• spills should be cleaned up immediately
with copious amounts of water
• contact with skin should be avoided
• in the event of contact, flush the area with lots
of water and have someone notify the
instructor