CHEM 253 UMW Bicarbonate & Carbonate Salts Lab Report

Description

I’ve attached the lab file in here along with my data that I took in the class while doing the experiment. Also, I’ve attached the class data in here. This lab report needs an abstract, results and discussion section, and an Excel spreadsheet. Please look over the files that I’ve attached, and let me know if you have any questions. Thank you.

Titration of a Mixture: bicarbonate & carbonate salts
2021)
Chem253 (Fall
Harris, “QCA” example problem on pg. 148, “Titration of a Mixture”
Read below – there’s a prelab assignment for you to do. Come prepared!
Overview
Titrating a mixture of sodium bicarbonate and sodium carbonate with hydrochloric acid involves
two reactions:
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
(1)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
(2)
Notice that the final endpoint is the same for both reaction 1 and 2 (look at the products), so if
we use a color changing indicator when titrating a mixture, we’ll measure the combined base
equivalents from the carbonate and bicarbonate. Now, let’s say we want to find out the amount
of bicarbonate in the mixture. If we know that the only components of the mixture are
bicarbonate and carbonate, we can use the approach taken by Harris in his example on page
148. However, if there are other components in the mixture (even if they aren’t basic) his
approach doesn’t work (why?). What can we do?
Here’s a possibility. Let’s say we know that bicarbonate is the only acid in the mixture (notice, it
has an acidic proton). In that case, we can react all the bicarbonate with standard base first:
HCO3-(aq) + OH-(aq) → CO32-(aq) + H2O(l)
(3)
If we use an excess of base, then we would have:
nOH- remaining = (nOH- added) – (nOH- reacted with bicarbonate)
(4)
Now we can do a back titration: we titrate the excess OH- with standard HCl and determine how
much was left unreacted. There’s a problem though – if we just back titrate, we’ll also titrate the
carbonate! So, we first remove carbonate by precipitation:
CO32-(aq) + Ba2+(aq) → BaCO3(s)
(5)
With the carbonate gone, we’re free to titrate the OH- without interference.
Some fine points that you need to think about in the back titration approach: How will you know
when you’ve added excess OH-? Might BaCO3(s) redissolve when you back titrate the OH-? If
so, how can you deal with that?
Prelab Assignment
You’ll be given a mixture of solid NaHCO3 and Na2CO3. So you know, it’s about 50% NaHCO3.
Determine a mass of the mixture to use so that you can reach the endpoint of a direct titration
with 0.1 M HCl with approximately 15 mL.
Procedure
Begin by setting up two back titrations to the point where BaCl2 is added. While the precipitate
settles, conduct two direct titrations.
Direct Titration

Add a carefully measured (and appropriate) amount of the mixture to a 125 mL Erlenmeyer
flask and dissolve in a minimal amount of DI water.

Add a few drops of bromocresol green and carefully titrate to the endpoint (blue to yellow,
but it may look green) using standard 0.1 M HCl.

Quickly boil the solution until it turns blue again – this gets rid of CO2, which is an acid and
interferes with the endpoint determination.

Finish the titration by carefully adding standard HCl until the solution just turns yellow
(probably just a drop or two of HCl).

Back Titration
• Add a carefully measured (and appropriate) amount of the mixture to a 125 mL Erlenmeyer
flask and dissolve in a minimal amount of DI water.

Add enough standard NaOH to completely react any bicarbonate (how much might this
be?).

Next, add about 1 mL of 1 M BaCl2 and mix thoroughly. Let settle for a while, then add a
drop or two of BaCl2 to verify that all the carbonate was precipitated.

Filter the solution through fine filter paper into a clean 125 mL Erlenmeyer flask.

Measure the volume of recovered solution and calculate the fraction of the original solution
you recovered. This will be needed in the mass percent calculations.

Add a few drops of phenolphthalein, and titrate the solution with standard 0.1 M HCl.

Data Analysis
In Lab (or very soon thereafter)

Record all of the calculations neatly in your notebook if doing calculations in lab. If not do
them on a separate piece of paper. – I will be checking them.

Using the approaches outlined above, calculate the moles of sodium bicarbonate in each
titration. Convert these into grams using the NaHCO3 molar mass (84.01 g mol-1).

Calculate mass% NaHCO3 using its mass and the measured sample masses.

Have the calculations checked by your instructor if you are in lab.
Post-Lab
Download the group data, and conduct a paired t-test to determine whether the two methods
(back and direct titration) give different results at the 95% confidence level. Treat each student
as a different “sample” and follow Harris’ example on page 77 in the text to conduct the t-test
using formulas in Excel. Check your work with the built in paired t-test analysis package in
Excel. Include both analyses in the spreadsheet you submit with the report.
Report
Your report should present the following data:

your data for the direct and back titration

your calculated mass % from the direct and back titration

a table of all the students’ mass % results for direct and back titration

the result of the t-test.
You must also be clear in the results section about what you infer from the results of the t-test.
Your report should clearly show equations for all chemical reactions (titrations, precipitations)
that you carried out. Refer to these equations as you discuss your results.
Student
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Mass % bicarbonate forward
Mass % bicarbonate back
49,48
42,42
18,4
86,7
52,95
52,14
19,15
-6,05
78,5
26,3
Here is some forward titration data you can use. The
molarity of the HCl was 0.09189 M
Mass solid Initial Buret
After 1st Before 2nd After 2nd
Total Volume
20.50 38.20
38.25
17.40
0.1029
3.15
0.1039
20.50
38.20
38.25
38.30
17.75
o
Pre-lab calculation?
o o.mot HCL tot) (15mt
mot
) ?
36.458 g
)
imal ha
Toolg
– s Empty 125 m Erlenmeyer flasks as 69469
Back 4 with Na co mixture as. 86 12 g
140
18.20mL mass of mixture before
22.00 mt 15 30mb filtration = 131.923
g
4.80 ml
mass mass of mixture after
filtration = 124.224g
20.10 h 16.90mL volume of mixture – 29.80mL
o
Pre-lab calculation?
o o.mot HCL tot) (15mt
mot
) ?
36.458 g
)
imal ha
Toolg
– s Empty 125 m Erlenmeyer flasks as 69469
Back 4 with Na co mixture as. 86 12 g
140
18.20mL mass of mixture before
22.00 mt 15 30mb filtration = 131.923
g
4.80 ml
mass mass of mixture after
filtration = 124.224g
20.10 h 16.90mL volume of mixture – 29.80mL