Orlando 1
Quantitative Analytical Chemistry
Joseph Orlando
Lab #8: Comparison Of Unknowns: Weak Acid Identification
Purpose:
The purpose of this experiment is to utilize two different methods; masking and ion exchange
resin conversion, to determine several metal ion concentrations. The ions that we are measuring
[H+], [Zn2+], [Mg2+], [Na+]. [H+] is measured by titration with standardized base. EDTA was used to
measure [Zn2+] and [Mg2+]. Cation exchange resin was used to replace all metal cations with H+.
My group
Procedure:
Standardization of NaOH
1. 4.0304 g NaOH were dissolved in 1L distilled water.
2. Approximately 0.511 g KHP was dissolved in 50 mL distilled water. A small amount of
phenolphthalein indicator was added. Three samples were assembled like this.
3. The NaOH solution was standardized using titration method.
[H+] Determination
4. [H+] of the unknown was determined by direct titration with the standardized base. 5 mL
unknown was diluted to 100 mL. 20 mL aliquots of this unknown solution were titrated using a
calibrated pH meter to determine equivalence point.
[Na+] Determination
5. An amount (enough for three titrations) of the unknown was passed through the cation
exchange resin to convert and replace all metal ions with [H+].
6. Three 20 mL aliquots of the converted unknown were titrated using phenolphthalein indicator
to determine equivalence point.
7. 0.3996 g CaCO3 was dissolved in 80 mL of 0.1 M HCl. Three 20 mL aliquots were taken out.
Enough buffer was added to each sample to reach a pH of 10.
[Zn2+] and [Mg2+] Determination
8. A 0.1 M EDTA solution was prepared by dissolving 14.3984 g EDTA in 500mL water. This was
standardized using the CaCO3 solution.
9. 5 mL of unknown was combined with 7 mL of buffer in an Erlenmeyer flask. 2 mL calmagite
indicator was added. Additional samples like this were prepared.
10. Each sample was titrated with the EDTA solution until a blue endpoint was noted.
11. 1.000 g of KCN was combined with 1 mL of unknown. Two samples were prepared.
12. Each sample was titrated with the EDTA solution.
Orlando 2
Data:
Standardization of NaOH
Trial #
Mass(g) KHP
NaOH(mL) Titrated
[NaOH]
1
0.0446
27.59
0.0227
2
0.0470
23.82
0.0246
3
0.0452
22.21
0.0286
Average
0.0456
24.54
0.0253
Titration of Unknown with NaOH
Trial #
mL of Unknown Used
NaOH(mL) Titrated
[H+]
1
20.00
-
0.000839
2
20.00
-
0.000879
3
20.00
-
0.000853
Average
20.00
-
0.000857
*Unable to collect NaOH(mL) Titrated data for Titration of Unknown with NaOH
Titration of Converted Unknown with NaOH
Trial #
mL of Unknown Used
NaOH(mL) Titrated
[H+]
1
20.00
18.50
0.0185
2
20.00
17.50
0.0175
3
20.00
17.50
0.0175
Average
20.00
17.83
0.0178
Orlando 3
Standardization of EDTA
Trial #
mL of CaCO3 Used
EDTA(mL) Titrated
[EDTA]
1
5.00
6.90
0.02890
2
5.00
6.70
0.02979
3
5.00
7.30
0.02795
Average
5.00
6.96
0.02869
EDTA Titration of Unknown
Trial #
mL of Unknown Used
EDTA(mL) Titrated
1
5.00
5.50
2
5.00
5.40
3
5.00
5.60
Average
5.00
5.50
[Zn2+] and [Mg2+]
EDTA Titration of Unknown and KCN
Trial #
KCN(g) used
1.00
Unknown(mL)
used
25.00
EDTA(mL)
Titrated
4.25
1
2
1.00
25.00
4.05
3
1.00
25.00
4.29
Average
1.00
25.00
4.19
[Mg2+]
Orlando 4
Ion
Average Concentration
H+
4.3𝑒 − 5 𝑀
Mg2+
0.0048 𝑀
Zn2+
0.0268 𝑀
Na+
−0.0138𝑀
Equations:
Amount of NaOH needed to make 0.1 M
0.1 𝑀 ×
39.995 𝑔 𝑁𝑎𝑂𝐻
× 0.500 𝐿 ≅ 2 𝑔 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
Amount of KHP needed for 25.0 mL Titration
. 025 𝐿 × 0.10 𝑀 𝑁𝑎𝑂𝐻 ×
1 𝑚𝑜𝑙 𝐾𝐻𝑃 204.23 𝑔 𝐾𝐻𝑃
×
= 0.511 𝑔 𝐾𝐻𝑃
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝐾𝐻𝑃
[NaOH]
0.0456g 𝐾𝐻𝑃 ×
1 𝑚𝑜𝑙 𝐾𝐻𝑃
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1
×
×
= 0.0253 𝑀 𝑁𝑎𝑂𝐻
204.23 𝑔 𝐾𝐻𝑃 1 𝑚𝑜𝑙 𝐾𝐻𝑃 0.02454 𝐿 𝑁𝑎𝑂𝐻 𝑎𝑑𝑑𝑒𝑑
[H+] (Titration)
0.01363 𝐿 𝑁𝑎𝑂𝐻 ×
.0253 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1𝐿
1 𝑚𝑜𝑙 𝐻+
1
× 1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻 × 0.020 𝐿 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑆𝑜𝑙. = 𝑀2 =0.000857 M
*Considering dilution of unknown solution . . .
𝑀1 =
(0.000857 𝑀)(.005 𝐿)
= 4.3𝑒 − 5 𝑀
. 100 𝐿
[H+] in Cation Exchange
0.01783 L 𝑁𝑎𝑂𝐻 ×
0.0253 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻
1 𝑚𝑜𝑙 𝐻 +
1
×
×
= 0.0178 𝑀
1𝐿
1 𝑚𝑜𝑙 𝑁𝑎𝑂𝐻 0.020 𝐿 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Orlando 5
Concentration of Cations ([Zn2+] + [Mg2+])
0.00550 𝐿 𝐸𝐷𝑇𝐴 × 0.02869 𝑀 𝐸𝐷𝑇𝐴
= 0.0316 𝑀
0.005 𝐿 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛
[Mg2+]
0.02869 𝑀 𝐸𝐷𝑇𝐴 × 0.00419 𝐿 𝐸𝐷𝑇𝐴 𝑎𝑑𝑑𝑒𝑑
= 0.0048 𝑀
0.025 𝐿 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛
[Zn2+]
(0.0316 𝑀) − (0.0048 𝑀) = 0.0268 𝑀
[Na+]
(0.0178𝑀) − (4.3𝑒 − 5𝑀) − (0.0048𝑀) − (0.0268𝑀) = −0.0138𝑀
Conclusion:
The purpose of this lab was to demonstrate two types of techniques that are commonly used for
selectivity in analytical chemistry. The two types of techniques that we focused on in this
experiment were masking and ion exchange resin. Using these two different techniques we
were able to calculate [H+], [Zn2+], [Mg2+], and {Na+]. The average calculated concentrations of
these cations are listed above in the data section. The only discrepancy in the data was the
calculated [Na+] value of -0.0138M. This may have been caused by human error in weighing
samples or determining volume titrated. The lab was a group effort between all groups in the
lab section which may have contributed to the amount of error. The calculated values seem
reasonable although due to the high chance of error, the actual values are unknown.