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”

Prelab Reading

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).

•

Report your data to your instructor.

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.

•

Report your data to your instructor.

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

added (g) Reading (mL)

Final Buret Reading (mL)

After 1st Before 2nd After 2nd

Total Volume

addition (L) addition (ML) addition (ml) added (mL)

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

Initial reading final reading Titration with Waolt

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

Initial reading final reading Titration with Waolt

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

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