Spectrophotometry

Question 1

What is spectrophotometry?

Answer 1

The use of light to study matter and determine concentration

-Measures the fraction of incident light transmitted through a solution.

Question 2

How does spectrophotometry work?

Answer 2

Different compounds absorb light at different wavelengths and the spectrophotometer can distinguish compounds

They are designed to transmit light of narrow wavelength ranges.

Question 3

Measurement of Absorbtion

Answer 3

All reactants appear colourless and therefor you need to find the wavelength where there is a difference in absorption between substrates and products.

To do this, plot a graph of absorbance vs wavelength between 240 and 400nm and using two products you calculate difference spectrum.

Question 4

How do you calculate the transmittance? (measure of light that passes through)

Answer 4

%T = I/Io times 100

I = intensity of transmitted light
Io = intensity of incident light

Question 5

What is beer-lambert law?

Answer 5

The proportion of incident light absorbed by a transparent medium is independent of the light intensity.

• Transmission of light decreases with increasing thickness of solution
• Absorbtion shows linear increase

Question 6

Beer Lambert Law

Answer 6

Transmittance and Absorbtion are inversely related, more absorbed less transmitted.
The relationship is logarithmic, if T is 50% then A is Log100.5

Question 7

How do you calculate Absorbance?

Answer 7

You calculate it by using Molar Extinction Coefficient

E = A/C times L

E= MEC (1.Mole-1cm-1)
A= Absorbance 
C = concentration (moles/l)
L = path length (cm)