Alternate Lab 4
Introduction:
Background Information: In this lab, we will be understanding how to calculate the
concentrations of the various things in weak acid-base systems, the Ka and Kb of the weak acids
or bases, the percentage error from the accepted values, and for the various salts, find the
dissociation and hydrolysis equations. We will also understand how to measure the pH of the salt
solutions.
Purpose: The purpose of this lab is to calculate Ka and Kb of the known concentrations of
weak acids and bases and the percentage error in the two parts. The purpose also includes the
relative pH of the various salt solutions and understand the results.
Procedure:
List of Materials: None
Step-by-Step Procedure: Part 1: 1. Using three separates, labeled sample cups, put a small
amount of vinegar, seltzer water, and ammonia into the sample cups.
2. Record the pH of the three liquids with the calibrated pH meter,
making sure to wash off the pH meter after each liquid. Put the values in your data table.
Part 2: Preparation: Label 4 of the sample cups with the chemical
formula for the four regents (NaCl, Na2CO3, (NH4)2CO3, and FeSO4). Label four of the
Erlenmeyer flasks with the same regent labels (NaCl, Na2CO3, (NH4)2CO3, and FeSO4). With
distilled water fill the wash bottle with it. Also fill the drinking glass half-way with the distilled
water. Make the solutions by adding one- fourth teaspoon into the labeled Erlenmeyer flask and
100 mL of distilled water.
Procedure: 1. The Erlenmeyer flasks with the 4 test solutions
should be lined up.
2. Find the pH of the test solutions using the pH meter. The
pH must stay steady for 20 seconds to be found as the actual pH of the solution. Rinse the meter
between the readings with the wash bottle.
3. Record the values in your data table.
Data:
Part 1 pH Data
Liquid
Type
Vinegar
pH
Level
2.3
Seltzer
Water
4.1
H+
5.01 x
10-3
7.94 x
10-5
OHN/A
N/A
A- Or
BH+
A- = 5.01
x 10-3
A- = 7.94
x 10-5
Molar
Concentrations
.833 M
0.0341 M (at
STP)
Ka or Kb
Ka= 3.03 x
10-5
Ka=1.85 x
10-7
pKa or
pKb
pKa=
4.52
pKa=
6.73
Ammonia
12.1
N/A
1.26 x
10-2
BH+ =
1.26 x 10-
2.94 M
Kb= 5.42 x
10-5
pKb=
4.27
2
Part 2 pH Level Data
Salt Solutions
NaCl
Na2CO3
(NH4)2CO3
FeSO4
pH Level
8.8
11.7
8.2
5.9
Discussion:
Analysis Questions:
Part I
Calculations:
1. Calculate the hydrogen ion concentration, [H+], for the two weak acids
(pH=-log[H+], or [H+]=antilog (-pH). If you have difficulty finding or
using the antilog function on your calculator, simply use this:
[H+]=10-pH . Record these in your data table.
2. Calculate the hydroxide ion concentration, [OH-], for the weak base
using this formula: pOH=14-pH, then [OH-]=antilog (-pOH) or [OH]=10-pOH. Record this in your data table.
3. Record [A-] or [BH+] in your data table. Refer to the information
provided in the introductory section of this lab.
4. The concentration of carbonic acid, which is the seltzer water, can be
found in a handbook of chemistry and physics and is 0.0341 M (at
STP). Record this in your data table.
5. You will need to calculate the molar concentrations of the vinegar as
well as ammonia. Both are industry standard 5.00% by mass solutions
in water. This means 5.00 g of solute/ 100.00 g of solution. Assume
that the solution is mostly water (which it is) and thus has a density of
1.00 g H2O/1.00 ml H2O. Now use this information plus the molar
masses of the vinegar (acetic acid) and ammonia (ammonium
hydroxide) to change the 5.00 g solute/100.00 g solution to M, which
is mol solute/L solution (H2O). Record these concentrations in your
data table.
6. Calculate the Ka or Kb using the formula given in the introduction.
Record these in your data table.
7. Calculate the pKa or pKb (pKa/b =-log Ka/b) and record these in your
data table.
Discussion Questions:
1. Find the Ka/b values in Appendices H and I (in Openstax) (Ka1 only)
and calculate the pKa/b values.
Answer: The Ka of vinegar is 1.8 x 10-5 at 25 ⁰C and the pKa value is
4.74. The Ka of seltzer water is 4.3 x 10-7 at 25 ⁰C and the pKa value is
6.37. The Kb of ammonia is 1.8 x 10-5 at 25 ⁰C and the pKb value is
4.74.
2. Calculate your percentage error.
Answer: My percentage error for vinegar is 4.64%. My percentage
error for seltzer water is 5.65%. My percentage error for ammonia is
9.92%.
3. Explain mathematically and chemically why you cannot measure or
calculate a pKa or pKb for a strong acid or strong base.
Answer: A strong acid or strong base completely ionizes in an aqueous
solution. This means that strong acids often give all of their protons to
the water so that there are no hydroxide ions which is needed to
calculate pKa. This also means that strong bases often accept so many
protons that there are no hydronium ions which is needed to calculate
pKa. Both strong acids and bases are missing something that is needed
to be able to mathematically figure out pKa or pKb.
Part II
Discussion
1. Salts can be formed by the reaction of an acid with a base. For each of
the four salts you tested, write the reaction of an acid with a base to
form that salt. Indicate whether the acid and base are strong or weak.
Answer: 1. NaOH (strong base) + HCl (strong acid) = NaCl
2. 2NaOH (strong base) + H2CO3 (weak acid) = Na2CO3
3. 2NH4OH (weak base) + H2CO3 (weak acid) =
(NH4)2CO3
4. Fe(OH)3 (weak base) + H2SO4 (strong acid) = FeSO4
2. Write the dissociation equations for each of the four salts (into ionsyou should use the subscript (aq) for them, such as
Ca(NO3)2(aq)→Ca2+(aq)+2NO3-(aq)).
Answer:
1. NaCl (aq) = Na+ (aq) + Cl-(aq)
2. Na2CO3(aq) = 2Na+(aq) +CO32-(aq)
3.(NH4)2CO3 (aq) → 2NH4+ (aq) + SO42- (aq)
4. FeSO4(aq) → Fe2+(aq) + SO42- (aq)
3. Write the hydrolysis equations for each of the four (4) salts (see intro
for examples).
Answer: 1. Na+ (aq) + H2O (l) ↔ H3O+ (aq) + Na (aq)
4.
5.
6.
7.
2. CO32-(aq) + H2O (l) ↔ H3O+ (aq) + CO3(aq)
3. NH4+ (aq) + H2O (l) ↔ H3O+ (aq) + NH4(aq)
4. Fe2+ (aq) + H2O (l) ↔ H3O+ (aq) + Fe(aq)
According to your data, indicate which salts appear to be neutral,
acidic, and alkaline.
Answer: NaCl, Na2CO3, and (NH4)2CO3 are alkaline. FeSO4 is acidic.
Explain why each salt appears to be neutral, acidic, or alkaline.
Answer: NaCl, Na2CO3, and (NH4)2CO3 all have pH levels over 7
while FeSO4 have a pH level under 7. Neutral is a pH of 7, acidic is a
pH under 7 while alkaline is a pH over 7.
List an example other than the salts used in this lab for each of the
following:
a. a neutral salt (formed by the reaction of a strong acid with a
strong base)
Answer: An example of a neutral salt is potassium nitrate.
b. a neutral salt (formed by the reaction of a weak acid with a
weak base)
Answer: An example of a neutral salt is lithium bromide.
c. an acidic salt
Answer: An example of an acidic solution is NaH2PO4.
d. an alkaline salt
Answer: An answer of an alkaline salt is KOH.
In the lab, which of the salts could be used to make a buffer and why?
Answer: Na2CO3 and (NH4)2CO3 could be used to make a buffer
because both salts are soluble in water which could help stabilize the
pH levels in the water solution.