Q: The energy required for a thymine-thymine (T-T) base pair in DNA to jump to a higher energy state is $$7.8 \times 10$$^{-19}$$ J. What is the wavelength $$\lambda$$ of light corresponding to this energy for one photon?

$$\lambda = \dfrac{hc}{E} = \dfrac{6.626 \times 10$$^{-34}$$ \times 3.00 \times 10^8$}{7.8 \times 10$$^{-19}$$} \approx 2.55 \times 10$$^{-7}$$ m

Q: If the energy of a photon is given by $$E = h \nu$$, and the frequency $$\nu of $$UVC light is $$1.2 \times 10$$^{15}$$ Hz, what is the energy of one photon?

$$E = 6.626 \times 10$$^{-34}$$ \times 1.2 \times 10$$^{15}$$ = 7.95 \times 10$$^{-19}$$ J

Question 1

Which of the following best explains why the discovery of new elements is still possible, but increasingly difficult as we move to higher atomic numbers on the periodic table?

Accepted Answer

The required nuclear reactions become more complex and the resulting elements are often highly unstable.

Question 2

In the context of wastewater treatment, which step is primarily responsible for physically removing bacteria from the water?

Accepted Answer

Filtration through sand or membranes

Question 3

A UV bulb is used in a wastewater treatment plant to kill bacteria. Which type of UV light is most effective for this purpose?

Accepted Answer

UVC ($$\lambda = 100-280 nm)$$

Question 4

The energy required for a thymine-thymine (T-T) base pair in DNA to jump to a higher energy state is $$7.8 	imes 10$$^{-19}$$ J. What is the wavelength $$\lambda$$ of light corresponding to this energy for one photon?

Accepted Answer

$$\lambda = \dfrac{hc}{E} = \dfrac{6.626 	imes 10$$^{-34}$$ 	imes 3.00 	imes 10^8$}{7.8 	imes 10$$^{-19}$$} \approx 2.55 	imes 10$$^{-7}$$ m

Question 5

Why are chlorine compounds (such as bleach) so effective at killing bacteria in water treatment?

Accepted Answer

Chlorine is highly reactive due to its high electronegativity and ability to disrupt cellular processes.

Question 6

During wastewater treatment, why might the effectiveness of UV disinfection vary depending on the concentration of microbes?

Accepted Answer

Higher concentrations of microbes can shield each other from UV light, reducing effectiveness.

Question 7

Which periodic table trend helps explain why chlorine is a strong oxidizing agent?

Accepted Answer

Chlorine has a high electronegativity and a strong tendency to gain electrons.

Question 8

If a wastewater treatment plant switches from chlorine disinfection to UV disinfection, which of the following is a likely advantage?

Accepted Answer

UV disinfection does not produce harmful chemical byproducts.

Question 9

Which of the following best describes the process by which UV light damages bacterial DNA?

Accepted Answer

UV light excites electrons in DNA bases, causing abnormal bonds such as thymine dimers.

Question 10

A new element is discovered with atomic number 120. Which of the following is most likely true about its chemical properties?

Accepted Answer

It will likely be highly unstable and radioactive.

Question 11

Which step in wastewater treatment is most directly responsible for converting organic material into fertilizer?

Accepted Answer

Digestion by bacteria

Question 12

If the energy of a photon is given by $$E = h 
u$$, and the frequency $$
u of $$UVC light is $$1.2 	imes 10$$^{15}$$ Hz, what is the energy of one photon?

Accepted Answer

$$E = 6.626 	imes 10$$^{-34}$$ 	imes 1.2 	imes 10$$^{15}$$ = 7.95 	imes 10$$^{-19}$$ J

Question 13

Which of the following is a reason why UVC light is preferred over UVA for disinfection in water treatment?

Accepted Answer

UVC photons have higher energy and are more effective at breaking chemical bonds in DNA.

Question 14

Which of the following best describes the historical trend in the discovery of new elements?

Accepted Answer

Most new elements discovered in recent decades are synthetic and not found in nature.

Question 15

A wastewater treatment plant is considering using both chlorine and UV disinfection. What is a potential benefit of this combined approach?

Accepted Answer

It can provide a broader spectrum of microbial inactivation, reducing the risk of resistant organisms.

Question 16

Which of the following is NOT a step in the typical municipal wastewater treatment process?

Accepted Answer

Electrolysis of water to produce hydrogen gas

Question 17

If a DNA base pair absorbs a photon and forms a thymine dimer, what is the most likely biological consequence?

Accepted Answer

Disruption of DNA replication and possible cell death

Question 18

Which of the following is a plausible reason why some bacteria survive UV disinfection in wastewater treatment?

Accepted Answer

Some bacteria are shielded by particles or biofilms, reducing their exposure to UV light.

Question 19

Which chemical property of chlorine makes it especially useful for disinfection in water treatment?

Accepted Answer

Its ability to accept electrons and form strong oxidizing agents

General Chemistry Study Notes: Elements, Water Treatment, and UV Disinfection

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