Experiment 3: Analysis of a Mixture of Carbonate and Bicarbonate
Andrew Mullenax
Objective:
The objective of this lab is to determine the amount of two different carbonate species via direct
and indirect titration methods.
Reactions:
𝐻𝐶𝑂3− + 𝐻 + → 𝐻2 𝐶𝑂3
𝐶𝑂32− + 2𝐻 + → 𝐻2 𝐶𝑂3
𝐻𝐶𝑂3− + 𝑂𝐻 − → 𝐶𝑂3− + 𝐻2 𝑂
𝐵𝑎2+ + 𝐶𝑂32− → 𝐵𝑎𝐶𝑂3
𝐵𝑎2+ + 𝑂𝐻 − → 𝐵𝑎(𝑂𝐻)2
Procedure:
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Obtain an unknown which contains a mixture of the carbonates of interest and store it in
the dessicator
Prepare approximately 0.1M HCl and approximately 0.1M NaOH using primary standard
grade KHP
Accurately weigh enough KHP to require at least 25mL of titrant
Use this to standardize your NaOH
Repeat until 3 good trials are obtained
Repeat using NaOH to standardize your acid
Weigh 2.0-2.5g of unknown into a 250mL Erlenmeyer flask. Dilute to the mark with
freshly boiled and triply distilled water
Pipet a 25.00mL aliquot of unknown into a 250mL Erlenmeyer flask and titrate with
standardized HCl using bromocresol green to determine the end point
Repeat until 3 good trials are obtained
Pipet a 25.00mL aliquot of unknown and a 50.00mL aliquot of standard NaOH into a
250mL Erlenmeyer flask
Add 10mL of 10wt% BaCl2, swirl to precipitate all BaCO3 then immediately titrate with
standard HCl using phenolphthalein indicator.
Repeat until 3 good trials are obtained
Data:
Volume of 6M HCl used = 16.6mL
Mass NaOH used = 2.00g
Mass KHP used = 0.51g
Bold = Bad trial
Standardization of NaOH
Trial 1
Mass KHP(g)
.5099
Trial 2
.5091
Trial 3
.5098
Trial 4
.5105
Average
0.5098
Volume NaOH
added(mL)
Concentration
of NaOH(M)
27.50
26.74
26.57
26.60
26.85
.0908
.0932
.0940
.0940
0.0937
Standardization of HCl
Volume HCl
Added(mL)
Volume
NaOH(mL)
Concentration of
HCl(M)
Trial 1
Trial 2
Trial 3
Average
25.13
25.11
25.13
25.12
25.0
25.0
25.0
25.0
.0932
.0933
.0932
0.0932
Mass Unknown = 2.0368g
Titration of Unknown(using bromocresol green)
Trial 2
Trial 1
Volume HCl
28.55
29.50
added(mL)
Moles of
Carbonate
.00266
.00275
and
Bicarbonate
Titration of Unknown(using phenolphthalein)
Trial 2
Trial 1
Volume HCl
29.33
29.80
added(mL)
Moles of
NaOH in
.00273
.00278
excess
Trial 3
Trial 4
Average
28.60
28.66
28.83
.00267
.00267
0.00269
Trial 3
Trial 4
Average
29.50
29.49
29.53
.00275
.00275
0.00275
Total Moles of NaOH = .004685
Results
Moles of
Bicarbonate
Trial 1
Trial 2
Trial 3
Trial 4
Average
Standard
Deviation
.00191
.00196
.00194
.00194
0.00194
.00002
Moles of
Carbonate
wt%
Bicarbonate
wt%
Carbonate
.00084
.00070
.00073
.00073
0.00075
.00006
5.72
5.87
5.81
5.81
5.8
.06185
2.47
2.06
2.15
2.15
2.21
.18007
Calculations:
Moles of KHP:
𝑀𝑎𝑠𝑠 𝐾𝐻𝑃 𝑥
. 5099𝑔 𝑥
1 𝑚𝑜𝑙
204.2212𝑔
1 𝑚𝑜𝑙
= .00250 𝑚𝑜𝑙𝑒𝑠 𝐾𝐻𝑃
204.2212𝑔
Concentration of NaOH:
𝑀𝑜𝑙𝑒𝑠 𝐾𝐻𝑃
1000𝑚𝐿
𝑥
𝑉𝑜𝑙𝑢𝑚𝑒 𝑁𝑎𝑂𝐻 𝑎𝑑𝑑𝑒𝑑
1𝐿
. 00250 𝑚𝑜𝑙𝑒𝑠 1000𝑚𝐿
𝑥
= .0908𝑀
27.50 𝑚𝐿
1𝐿
Concentration of HCl:
𝑣𝑜𝑙𝑢𝑚𝑒 𝑁𝑎𝑂𝐻 𝑥 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑁𝑎𝑂𝐻
𝑣𝑜𝑙𝑢𝑚𝑒 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑
25.0𝑚𝐿 𝑥 .0937𝑀
= .0932𝑀
25.13𝑚𝐿
Moles of Carbonate and Bicarbonate:
𝑉𝑜𝑙𝑢𝑚𝑒 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑 𝑥 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑜𝑓 𝐻𝐶𝑙
1000
29.50𝑚𝐿 𝑥 .0932𝑀
= .00275 𝑚𝑜𝑙𝑒𝑠
1000
Moles of excess NaOH:
𝑉𝑜𝑙𝑢𝑚𝑒 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑 𝑥
29.80𝑚𝐿 𝑥
1𝐿
. 0932𝑚𝑜𝑙
𝑥
1000𝑚𝐿
1𝐿
1𝐿
. 0932𝑚𝑜𝑙
𝑥
= .00278 𝑚𝑜𝑙𝑒𝑠
1000𝑚𝐿
1𝐿
Total Moles of NaOH:
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑁𝑎𝑂𝐻 𝑥 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑁𝑎𝑂𝐻
. 05𝐿 𝑥 .0937𝑀 = .004685 𝑚𝑜𝑙𝑒𝑠 𝑁𝑎𝑂𝐻
Moles of NaOH reacted:
𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑒𝑠 𝑁𝑎𝑂𝐻 − 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑒𝑥𝑐𝑒𝑠𝑠 𝑁𝑎𝑂𝐻
. 004685 𝑚𝑜𝑙𝑒𝑠 − .00278 𝑚𝑜𝑙𝑒𝑠 = .00191 𝑚𝑜𝑙𝑒𝑠 𝑁𝑎𝑂𝐻
Moles Carbonate:
𝑀𝑜𝑙𝑒𝑠 𝐶𝑎𝑟𝑏𝑜𝑛𝑎𝑡𝑒 𝑎𝑛𝑑 𝑏𝑖𝑐𝑎𝑟𝑏𝑜𝑛𝑎𝑡𝑒 − 𝑀𝑜𝑙𝑒𝑠 𝑁𝑎𝑂𝐻 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
. 00275𝑚𝑜𝑙𝑒𝑠 − .00191𝑚𝑜𝑙𝑒𝑠 = .00084𝑚𝑜𝑙𝑒𝑠 𝐶𝑂32−
Wt% bicarbonate:
𝑚𝑜𝑙𝑒𝑠 𝐻𝐶𝑂3− 𝑥 61𝑔/𝑚𝑜𝑙 𝑥
. 00191 𝑚𝑜𝑙𝑒𝑠 𝑥
1
𝑥 100
𝑚𝑎𝑠𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
61𝑔
1
𝑥
𝑥 100 = 5.72%
1 𝑚𝑜𝑙 2.0368𝑔
Wt% carbonate:
𝑚𝑜𝑙𝑒𝑠 𝐶𝑂32− 𝑥
. 00084 𝑚𝑜𝑙𝑒𝑠 𝑥
60𝑔
1
𝑥
𝑥 100
1 𝑚𝑜𝑙 𝑚𝑎𝑠𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
60𝑔
1
𝑥
𝑥 100 = 2.47%
1 𝑚𝑜𝑙 2.0368𝑔
Conclusions:
The purpose of this lab was to determine the relative weight percentage of carbonate and
bicarbonate in an unknown sample. The average weight percentages of bicarbonate and
carbonate were determined to be 5.80% and 2.21% respectively.
Post-lab Questions:
A. A primary standard is a pure/stable reagent that can be used directly after weighing.
B. A secondary standard is a standard that has been standardized using a primary standard.
C. An indirect titration is one in which an excess of standard reagent is added to the analyte
and is titrated endpoint.
D. The titrant is a reagent of known concentration used to titrate the unknown unknown.