# 0.3 Determine the value of an equilibrium constant by complex ion  (Page 2/2)

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A = $\epsilon$ bc Equation 4

In this equation, the measured absorbance (A) is related to the molar absorptivity constant ( $\epsilon$ ), the path length (b), and the molar concentration (c) of the absorbing species. The concentration is directly proportional to absorbance.

Thiocyanate ( ${\text{SCN}}^{-}$ ) is an interesting ion and is widely used in a variety of industrial processes such as the manufacturing of thiourea, photofinishing, metal separation, and electroplating. It is also found in gold mining wastewater as a result of treating the cyanide rich ore with sulfur dioxide in order to produce the less toxic thiocyanate( ${\text{SCN}}^{-}$ ) ion. Iron, as you will see later on in the semester, has the unique ability to inexpensively clean up and produce drinking quality water in Third World countries.

## Equipment

• 5 test tubes
• Burette (0.1 ml graduations) filled with 0.0200M $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ in 0.5 M  ${\text{HNO}}_{3}$
• Neighboring partners’ burette filled with $6\text{.}\text{00}×{\text{10}}^{-4}$ M KSCN
• stirring rod
• small labels or china clay pencil
• 5 cuvettes for the spectrophotometers
•  250 ml bottle acetone for rinsing

## Hazard: as always wear safety glasses while performing this experiment

Contamination Notes: If your flask is wet before you prepare your standard/sample solutions ensure that the flask is wet with dilutant (in this case it is 0.5 M HNO 3 ) .

Calibration Of MicroLab/Spectrophotometer

N.B. Do not use test tubes from your drawer!

Find and open the MicroLab program.

1. Find and open the MicroLab program. This brings up a box that will enable you to select from a list of experiments. Select colorimeter. Check that the accompanying box has power and is turned on, and that it is connected to the laptop via the USB plug.
2. In the tab labeled “New” you will find the icon for the “Spectrophotometer”, please double click this. Make sure that you click on the absorbance tab.
3. This brings up the program, at which point you should take a reading of a blank, this is done by filling a vial with 15 mL of 0.5 M  ${\text{HNO}}_{3}$ (not deionized water in this case) and placing in the appropriate slot. Covering with the film case to exclude ambient light from entering the system. When the blank sample is in place, click the button “Read Blank”. This will generate a series of data points.
4. A solution of 0.0200M $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ in 0.5 M  ${\text{HNO}}_{3}$ has been prepared for you.
5. Dilute 1.5, 3.0, 4.5 and 6.0 mL portions of $6\text{.}\text{00}×{\text{10}}^{-4}$ M KSCN to 20 mL with the 0.0200M $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ in 0.5 M  ${\text{HNO}}_{3}$ . For this you can use some of the smaller beakers in your lab drawer. These need to be rinsed and dried thoroughly before each use. Use two clean, dry burettes to dispense the two solutions.
6. This will give you 4 solutions which can be assumed to be $6\text{.}0×{\text{10}}^{-5}$ M, $1\text{.}2×{\text{10}}^{-4}$ M, $1\text{.}8×{\text{10}}^{-4}$ M and $2\text{.}4×{\text{10}}^{-4}$ M in ${\text{FeSCN}}^{2+}$
7. Take the most concentrated solution and collect a visible spectrum from 400 to 800 nm to determine ${\lambda }_{\text{max}}$ .
8. Measure the absorbance of all the solutions at ${\lambda }_{\text{max}}$ , using 0.5 M  ${\text{HNO}}_{3}$ as the blank/reference solution.
9. Measure the absorbance of these solutions again at 430nm, under the same conditions.
10. Plot absorbance vs. [ ${\text{FeSCN}}^{2+}$ ] for both wavelengths using Excel and add a linear trendline passing through the origin (under Options, set intercept equal to 0). Using Beer’s law and the equation of the trendline, find the molar absortivity.

## Experimental determination of kc

The mixtures will be prepared by mixing solutions containing known concentrations of iron (III) nitrate, $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ , and potassium thiocyanate, KSCN. The color of the ${\text{FeSCN}}^{2+}$ ion formed will allow us to determine its equilibrium concentration. Knowing the initial composition of a mixture and the equilibrium concentration of ${\text{FeSCN}}^{2+}$ , we can calculate the equlibrium concentrations of the rest of the pertinent species and then determine ${K}_{c}$ .

1. Label five regular test tubes 1 to 5, with labels or by noting their positions in the test tube rack.
2. Dilute 0.02 M $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ in 0.5 M ${\text{HNO}}_{3}$ by a factor of 10 using the volumetric flask that you have in your drawer. Remember to do the dilution with 0.5 M ${\text{HNO}}_{3}$ not water.
3. Pour about 25 mL 0.002 M $\text{Fe}\left({\text{NO}}_{3}{\right)}_{3}$ in 0.5 M ${\text{HNO}}_{3}$ into a clean, dry burette.
4. Dispense 5.00 mL of that solution into each test tube.
5. Then pour about 20 mL $6\text{.}\text{00}×{\text{10}}^{-4}$ M KSCN into another clean, dry burette.
6. Dispense 1,2,3,4, and 5 mL from the KSCN burette into each of the corresponding test tubes labeled 1 to 5.
7. Using a small graduated cylinder, dispense the proper number of milliliters of 0.5 M ${\text{HNO}}_{3}$ into each test tube to bring the total volume in each tube to 10.00 mL.
8. The volumes of reagents to be added to each tube are summarized in the table.
 TestTube # 1 2 3 4 5 Reagents (mL) 1 2 3 4 5 Fe(NO 3 ) 3 5 5 5 5 5 KSCN 1 2 3 4 5 0.5 M HNO 3 4 3 2 1 0
1. Mix each solution thoroughly with a glass stirring rod. Be sure to dry the stirring rod after mixing each solution to prevent cross-contamination.
2. Place the mixture in tube 1 in a spectrophotometer cell and measure the absorbance of the solution at ${\lambda }_{\text{max}}$ .
3. Determine the concentration of ${\text{FeSCN}}^{2+}$ from your calibration curve. Record the value on your Report form. Repeat the measurement using the mixtures in each of the other test tubes.

#### Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
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Abigail
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Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, General chemistry lab spring. OpenStax CNX. Apr 03, 2009 Download for free at http://cnx.org/content/col10506/1.56
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