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Chemistry 1516L Week 5

Graduate Teaching Assistant

Ward Beecher Hall

Laboratory Computer Use

http://www.ysu.edu/thecode.pdf

The computers in the YSU General Chemistry labs are to be

used only for school-related activities and to help you with

experiments

They are not to be used for surfing inappropriate, non-school-

related websites; that would be a direct contravention of the

on-campus YSU code of conduct

Since you log in with your student email ID, whatever you

look at during class is stored under your account, which we

have access to

For further details, see:

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Pre-Lab Material – GET IT AND READ IT

Background Material – 1516L Webpage

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Determination of an Equilibrium Constant

Fe3+ + -SCN FeSCN 2+

Combine these two solutions

(We know their concentrations)

Measure how much of this is formed by spectroscopy (i.e. absorbance of light)

A = εεεε • b • [FeSCN2+]

i.e. a Linear relationship between A and [conc’n]

K (equilibrium constant) = [FeSCN2+]

[Fe3+]eq • [SCN–]eq

You can calculate [Fe3+]eq and [SCN–]eq by subtraction

The Experiment – Application of Beer’s Law

5 samples needed;

1 blank (water), 4 of different concentrations of Fe3+ and –SCN in 1 M HClO4

solution. Prepare as directed in text.

• Measure Transmittance/Absorbance using MicroLab

• Apply those numbers to Beer’s Law to get concentrations of [FeSCN2-] in

each vial

A = εεεε • b • [FeSCN2+]

εεεε = 3494 L mol-1 cm-1 ; b = 2.5 cm

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Calculations – need to know [Fe3+]o and [-SCN]o

Before you begin taking readings:

Need to calculate initial concentrations of

[Fe3+]o and [FeSCN2+]o in each vial – add to report sheet

Taking readings:

Spectrophotometer cavity

Setting the samples in the Spectrophotometer

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Sample cover applied – ready to collect data

The MicroLab interface before readings

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After taking a reading - %T at different wavelengths

After taking a reading - A at different wavelengths

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After all four readings

You should have:

• 4 sets of data, you need %T as well as A for the report sheet

• Data to be used are at the Wavelength of HIGHEST absorbance (click on

that bar in the Spectrum Profile)

• Check to see shape of the plot of %T versus Concentration

• Change this to A versus Concentration to validate Beer’s Law

A = e • b • [FeSCN2+]

• Read the corresponding web page material to see if there are any

modifications or suggestions for that experiment. This will save you

time in lab.

• Bring goggles, lab coat, paper towels, dishwashing gloves, handout

from website.

• Email me with any concerns.

For Next Lab Session: