3. Why does absorbance increase with concentration? 4. What is happening on the

Question

3. Why does absorbance increase with concentration? 4. What is happening on the atomic level when a photon of light is absorbed? 5. Use Beer's Law to explain why you should ideally use the same cuvette for all absorbance measurements. Hint: remember path length 6. Determine the value of e (including units) for the solution. Hint: the slope of the line of best fit is equal to eb, and b = 1 cm. 7. In 1913 Niels Bohr proposed the Bohr model of the atom. His model was based on the line spectra of the hydrogen atom. Explain what is correct about the Bohr model and what is ncorrect. 8. Bonus**Why is the absorbance spectra of a molecule very broad, while atoms have line spectra?

Explanation / Answer

ans 2 . Absorbance = e L c

Where A is absorbance
e is the molar absorbtivity with units of L mol-1 cm-1
b is the path length in centimetres.
c is the concentration of solution, expressed in mol L-1

as absorbance is directly proportional to concentration . So if the concentration increases absorbance also increses.

ans 4

When a specific energy photon is absorbed by the atom, this absorption occurs at a specific wavelength in the spectrum.
Atom goes into "excited" state. The electron eventually comes back to the ground state. However, the atom lose energy to do this, and so it releases a photon of the same energy as the one it absorbed . This process is called emission .
The atom might have absorbed another photon to gain energy so as to jump up to another energy level. If a photon with a sufficiently large energy gets absorbed, it cause an electron to unbound from its nucleus, which is called  ionization.

ans 5 .  while computing the concentration spectrophotometry, one uses the Beer's Law

Absorbance = e L c

concentration = A/Le. Since e will be the same for one substance .L also should be constant . To have L constant, y same cuvette should be used for all absorbance measurements. Different cuvette might have different thickness, shape, etc, giving different L values.

ANS 6

IN absobance v/s concentration graph

slope = e L

therefore e= slope /L = Slope /1 cm

ans 7.

Bohr atomic model :

Defects of Bohr’s theory :

ans 8

In atoms the electrons jumps up by gaining the energy from photons and later emit the energy to give line spectra . Each element has a different set of allowed orbits, so each element emits or absorbs photons with different energies .

Molecules also produce spectral lines, but their spectra are much more complex than the spectra of single atoms, and and thus many bands merge to show broad bands instead of narrow lines.

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