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1. pH Using Indicators
This part is in a separate document “pH
using indicators” in the Bridges folder for this week.
Fill in the pH from the observed indicator
color. You will have to use less than
(<) or greater than (>) in most cases.
|
Litmus |
Btb |
mp |
mr |
bcg |
mo |
bpb |
Tb |
cr |
mg |
Mv |
|
Litmus
paper |
Brom
Thymol Blue |
Methyl
orange |
Methyl
Red |
Brom
Cresol Green |
Methyl
orange |
Brom
Phenol Blue |
Thymol
Blue |
Cresol
Red |
Green
color |
Violet
|
|
4.5 (red) - 8.3 (blue) |
6.
0 (yellow) - 7.6 (blue) |
4.4 (red)
- 6.2 (yellow) |
4.4 (red)
- 6.0 (yellow) |
4.0 (yellow) – 5.6 (blue0 |
5.2 (red)
- 6.6 (yellow) |
3.0 (yellow)
- 4.6 (blue) |
1.2 (red) -
2.8 (yellow) |
7.2 (pink/orange) – 8.8 (red/purple) |
> 11 strong alkali |
>11 violet or indigo |
Unknown Number _____12_____
pH of UNK _________strong alkali______
2. Making pH
measurements
pH of Salt Solutions
Record the name, formula and pH of each salt solution. If
the pH is not neutral, for the ions that undergo hydrolysis, write down the
hydrolysis reaction and indicate which species is responsible for the observed
pH. it.
3. Determination of Ka for a Weak Acid
pH of Half Neutralized Acid: _________5.2______
pH of the half-neutralized acid is the pH level when half of the acid has been neutralized. The steep portion of the curves shows the equivalence point and it occurs until a pH of around 10. Consider that the weak acidic solution is having concentration of 0.6M and the pH level is 5.2.
From the observed pH, calculate [H+]
of the solution and Ka of
the weak acid.
Show your reasoning and your work for your determination of
Ka from the measured pH.
4. Properties of a Buffered Solution
|
Place
25 mL of the unknown half neutralized acid buffer mixture in a small beaker
and measure its pH with the meter. Add 5 drops of 0.1 M strong acid HCl to
the buffered mixture and stir. Allow the electrode to equilibrate and record
the pH. The pH should drop only by a small fraction of a pH unit (if at all).
|
|
|
Write the net
ionic equation demonstrating why an HA/A- buffered system
resists changes in its pH
when strong acid (H+) is added. Considering the pH level
and the concentration, the electrode records the equilibrium. Adding the acid
reduced pH level of the solution. Conjugate acid neutralizes the strong base
by adding the acid.
For the buffer to resist
the changes in the level of pH, the adding conditions of
|
|
|
Place
another 25-mL sample of the buffered mixture in a small beaker. Add 5 drops
of 0.1 M strong base NaOH to the buffered mixture and stir. Record the pH
after allowing the electrode equilibrate in the solution. The pH should have
risen only by a small fraction of a pH unit (if at all).
|
|
|
Write net ionic equations demonstrating why an HA/A- buffered system
resists changes in its pH
when strong base (OH-) is added.
The pH values of the
solution and individual component does not change as
|
|
5. Conductivities
of Acid, Base, and Salt Solutions
Record your observations and explain why the measured conductivity for the HCl and NaOH solutions is greater than for the acetic acid and ammonia solutions?
The conductivity values of the
solution depend on the percentage weight of the component in the solution. The
maximum conductivity of ammonia solution at temperature of 25 depends on the
molar weight of the solution. considering the temperature constant at 25
degrees the conductivity of solution depends on the values of 
. The 
of HCl, NaOH, acetic acid and ammonia are 617, 1780, 3390, and 1200/5.5%
respectively. The higher the value of 
the lower will be the value
of conductivity of the solution.
There is no formal report for this experiment. Just turn in your notebook pages at the end
of the lab period.
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