Stacey Snyder
4th Hour
3/28/11
pH Lab
Introduction:
Making chocolate milk can be difficult. Sometimes there is too much milk or too much chocolate. In
order for it to taste the best possible the equivalence point needs to be found. The equivalence point in
this situation would be when the milk to chocolate ratio is when there is an equal consistency of
chocolate in the milk. But in chemistry the equivalence point is when an acid and base are mixed
together and there is a big jump showing that the two substances have a similar volume when they are
similar molarities. The equivalence point is easily visible when the titrations are put into a chart and
graphed. This graph is called the titration curve. With the titration curve, the equivalence point can be
determined more precisely. With equivalence point the Ka or Kb can be determined if molarity and
starting volume is know. If an acid and a base are titrated with the volumes and molaritys of both are
known or can be determined then Ka or Kb can be determined.
Materials and Methods:
Obtained a ring stand, 2 burettes, and a clamp to hold the burettes above a beaker and was then set up.
One burette was taken off the apparatus and cleaned, then rinsed with vinegar. After the burette was
rinsed 15mL of vinegar was measured out and then dripped into the clean beaker, the empty burette
was then placed back into the apparatus. The other burette was then taken down from the apparatus
and cleaned, then rinsed with NaOH. Then after being rinsed with NaOH, more NaOH was added to fill
the burette and start titrations. Then a calculator with a pH probe was obtained and placed into the
beaker with vinegar. NaOH was added to the beaker in increments of ½ a mL, after each addition the pH
was recorded. This was repeated until a jump occurred in the pH and then continued for 35 more
additions. The apparatus was left not cleaned and the beaker was dumped.
Stacey Snyder
4th Hour
3/28/11
Results:
Titration curve of strong acid vs strong base
14
12
10
Trial 1
8
pH
Trial 2
Trial 3
6
Trial 4
Trial 5
4
Trial 6
2
0
0
10
20
30
Volume of base added
40
50
60
Stacey Snyder
4th Hour
3/28/11
derivative of DpH over DV plot against average
V(SA and SB)
18
16
14
12
Trial 1
DpH/DV
10
Trial 2
Trial 3
8
Trial 4
6
Trial 5
Trial 6
4
2
0
0
10
-2
20
30
40
50
average volume
Strong Acid and Strong Base
Volume at the
Equivalence
pH at Equivalence
Initial
Trial
Point
Point
Volume Molarity
1
30.75
8.4
25
0.3075
2
13.75
9.6
10 0.34375
3
25.75
9.5
20 0.321875
4
43.25
6.3
35 0.308929
5
38.25
7.8
30 0.31875
6
18.25
6.7
15 0.304167
Average pH
8.05
Standard Deviation
1.38383525
60
61.75, 0.04
0
70
Stacey Snyder
4th Hour
3/28/11
Titration Curve of Weak Base vs Strong acid
16
14
12
10
pH
pH
pH
8
pH
pH
pH
6
pH
4
2
60, 0.82
0
60, 0
0
10
20
30
40
Volume of acid added
50
60
70
Stacey Snyder
4th Hour
3/28/11
derivative of DpH over DV plot against average
V(SA and WB)
14
12
10
Trial 1
DpH/DV
8
Trial 2
Trial 3
6
Trial 4
Trial 5
4
Trial 6
2
0
59.75, 0
0
-2
10
20
30
Average Volume
40
50
60
Stacey Snyder
4th Hour
3/28/11
Strong Acid and Weak Base
Volume at the
peak change in
pH from second
graph
Trial
pH from first graph
that matches with
volume
Initial
volume
Molarity
1/2 Eq.
Pt.
pH of
1/2 Eq.
Pt.
1
11.75
6.5
12 0.244792
5.875
9.48
2
21.75
4.2
20 0.271875
10.875
9.7
3
6.75
10.4
0.16875
3.375
13.4
4
35.75
5.2
35 0.255357
17.875
9.57
5
26.75
4.7
30 0.222917
13.375
9.5
22.25
4.5
5.916666667
2.340441554
25
11.125
9.54
6
Average pH
Standard Deviation
10
0.2225
Average Kb
Standard Deviation
TRUE
*trail 3 was dropped for averages and standard deviations calculated.
Kb
3.31131E10
1.99526E10
3.98107E14
2.69153E10
3.16228E10
2.88403E10
2.80888E10
5.1447E-11
1.8*10^-5
Stacey Snyder
4th Hour
3/28/11
Titration Curve of Weak Acid vs Strong Base
14
12
10
Trial 1
8
pH
Trial 2
Trial 3
Trial 4
6
Trial 5
Trial 6
4
Trial 7
2
0
0
10
20
30
40
Volume of base added
50
60
70
Stacey Snyder
4th Hour
3/28/11
derivative of DpH over DV plot against average V
(WA and SB)
9
8
7
6
Series1
Series2
DpH/DV
5
Series3
4
Series4
Series5
3
Series6
2
Series7
1
0
0
5
-1
10
15
20
25
30
Average Volume
Weak Acid and Strong Base
Volume at
pH at
pH at
Equivalence
Equivalence
Initail
1/2 Eq.
1/2 Eq.
Trial
Point
Point
Volume Molarity Pt.
Pt.
Ka
1
12.25
8.8
15 0.204167
6.125
4.53 2.95121E-05
2
10.75
8.3
15 0.179167
5.375
4.6 2.51189E-05
3
23.25
8.2
27.6 0.210598
11.625
4.7 1.99526E-05
5
12.25
13
10 0.30625
6.125
4.6 2.51189E-05
6
15.75
9.15
20 0.196875
7.875
4.6 2.51189E-05
7
22.75
7.96
25
0.2275
11.375
4.62 2.39883E-05
Average pH
9.235
Standard Deviation
1.894346853
Average Ka
2.48016E-05
Standard Deviation
3.05499E-06
TRUE
1.7*10^-5
*Trial 4 was omitted from the table due to the fact that it was a copy of trial 3
Stacey Snyder
4th Hour
3/28/11
Calculations:
Finding molarity of NaOH from 5 grams and 500mL.
5g NaOH
1mol NaOH
40g NaOH
=.25M of NaOH
.5 L NaOH
Finding Molarity of HCl
11.75mL NaOH
.25mol NaOH
1L
1mol HCl
1mol NaOH
= .2448M HCl
12mL
Finding Ka from pKa
Ka =10^-pKa
Ka=10^-9.48
Ka= 3.31131E-10
Conclusions:
In the titration between a strong acid (HCl) and a strong base (NaOH), the equivalence point should have
given a pH of around 7, however as show in the graph and table for SA and SB this was not the case. The
average pH of the equivalence point is about 8.05. This may be due to lack of accuracy when adding the
base, possibly adding too much at a time. It is off by a little bit so this could potentially become a
problem. In the second titration between a strong acid and weak base the average pH was calculated
to be about 5.92 at the equivalence point. The standard deviation of the pH was roughly 2.3, making
the trials not extremely precise. If needed to determine Kb would alter, therefore not making the trial
very accurate. This could be from miss reading on the graphs or poor lab technique. This was after the
3rd trial was dropped because it was so far off from the other trials. The Kb was calculated to be
2.80888E-10 and the actual is 1.8 *10^-5. Basically this is terrible; the two Kb’s are very different. In the
last titration at the equivalence point the average pH was found out to be about 9.24. Again the
standard deviation was roughly 2, making these trials also inaccurate. This could be from miss reading
on the graphs or poor lab technique. Trial 4 was dropped from the calculations because it was a
duplicate of trail 3 with a typo somewhere in the original data table. The Ka calculated from this lab is
2.48016E-05 and the actual is 1.7 X10^-5. The two Ka’s are closer that the Kb’s but still not very
accurate. Due to poor lab technique, this would not be a good idea for determining an unknown acid or
base. The probes were jumping a lot with the addition of NaOH and could have been miss read or the
charts and graphs would have been interpreted poorly causing everything from then to be altered and
Stacey Snyder
4th Hour
3/28/11
miss leading. The lab could have been conducted better there the graphs were enlarged and measured
to the nearest thousandth place and if the trial were conducted more precisely with the ½ mL additions.
This is kind of similar to making coffee, when to many grounds are added to the filter the coffee turns
out strong. Or if not enough grounds are added the coffee is weak. In the case of this lab, the coffee
would have been strong for the third titration, weak for the second and a little strong for the first, but
the best of the three choices.