Physical Chemistry : Physical Chemistry

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #71 : Physical Chemistry

Two light sources are placed equidistant from a detector. The light from light source A is detected quicker than light from light source B. What can be concluded from these results?

Possible Answers:

Source A could produce red light whereas source B could produce blue light

The number of photons per unit time is higher from source A

Source A has a higher wavelength

The results are invalid

Correct answer:

The results are invalid

Explanation:

The question states that there are two light sources and that both are placed equidistant from a detector. This means that any differences in detection speed is not related to the distance. The question also states that the detector picks up light from source A quicker that light from source B. This means that the speed of the light from source A is faster. Recall that light (or electromagnetic rays) can have varying wavelengths and frequencies; however, the speed of all electromagnetic rays is the same. It doesn’t matter if it is gamma rays or visible light, the speed of light in air is . The information given in this question contradicts this; therefore, these results are invalid.

We cannot determine the color of the light for the light sources because we don't have any information regarding their respective wavelength or frequency. The number of photons per unit time refers to the amplitude of the wave, which also cannot be determined from the given information.

Example Question #5 : Electromagnetic Spectrum And Radiation

What is the frequency of microwave radiation that has a wavelength of 0.885m?

Possible Answers:

None of these

Correct answer:

Explanation:

To solve this problem, we use the following equation:

Where  is the wavelength and  is the frequency.

Wavelength is given, and as we are dealing with the electromagnetic spectrum, we know that  is equal to the speed of light, which is given. We need to solve for frequency, so we arrange the equation as follows: 

Plug in known values and solve.

Example Question #1 : Electromagnetic Spectrum And Radiation

The wavelength of a particular color of orange light is 629nm. 

What is the frequency of this color?

Possible Answers:

Correct answer:

Explanation:

To solve this problem, we use the following equation:

Where  is the wavelength and  is the frequency.

Wavelength is given, and as we are dealing with the electromagnetic spectrum, we know that  is equal to the speed of light, which is given. We need to solve for frequency, so we arrange the equation as follows: 

Plug in known values and solve.

Example Question #7 : Electromagnetic Spectrum And Radiation

A local FM radio station broadcasts at a frequency of 99.0MHz.

What is the wavelength at which the station is broadcasting?

Possible Answers:

Correct answer:

Explanation:

To solve this problem, we use the following equation:

Where  is the wavelength and  is the frequency.

Rearrange the equation to solve for wavelength.

Plug in known values and solve.

Example Question #11 : Quantum Chemistry

How many sublevels are there with n = 2 in an atom?

Possible Answers:

12

20

16

18

9

Correct answer:

18

Explanation:

The types of subshells, from smallest to largest, are as follows: s, p, d, and f. These four subshells correspond respectively to the following quantum numbers: 0, 1, 2, and 3. The total number of orbitals in an atom with n = 3 is 32 or 9: one 3s , three 3p and five 3d. Each orbital can hold a maximum of 2 electrons. Therefore, the maximum number of electrons possible is 2 x 9 = 18.

Example Question #12 : Quantum Chemistry

What is the maximum number of electrons possible with n = 4 in an atom?

Possible Answers:

Correct answer:

Explanation:

The types of subshells, from smallest to largest, are as follows: s, p, d, and f. These four subshells correspond respectively to the following quantum numbers: 0, 1, 2, and 3. The total number of orbitals in an atom with n = 4 is 42 or 16: one 4s , three 4p , five 4d and seven 4f. Each orbital can hold a maximum of 2 electrons. Therefore, the maximum number of electrons possible is 32. 

Example Question #1 : Quantum Chemistry

Which of the following sets of quantum numbers () could correspond to a  orbital?

Possible Answers:

Correct answer:

Explanation:

Since we're looking at the  orbital, we know . The range of possible values for  is 0 to . Possible values for  range  to . Therefore, among the answer choices,  is the only possible combination of quantum numbers corresponding to an  orbital. 

Example Question #2 : Quantum Chemistry

Which of the following sets of quantum numbers could correspond to a  orbital?

Possible Answers:

Correct answer:

Explanation:

Since we're looking at the  orbital, we know . The range of possible values for  is 0 to . Possible values for  range  to . Therefore, among the answer choices,

is the only possible combination of quantum numbers corresponding to a  orbital.

Example Question #3 : Quantum Numbers

What can be concluded about the quantum numbers for potassium and potassium ion?

Possible Answers:

All valence electron(s) of potassium ion have a different orbital angular momentum quantum number than valence electron(s) of potassium

More than one of these

Valence electron(s) of potassium ion have higher energy

Valence electron(s) of potassium has a higher principal quantum number than valence electron(s) of potassium ion

Correct answer:

Valence electron(s) of potassium has a higher principal quantum number than valence electron(s) of potassium ion

Explanation:

Potassium has one valence electron. This means that there is one electron in its outermost shell (4th shell). Potassium ion, on the other hand, loses an electron and has a complete octet (has eight valence electrons) in its 3rd shell. Recall that the principal quantum number signifies the shell. Since the valence electron of potassium is found in the fourth shell, . Similarly, the valence electrons of potassium ion are found in the third shell and  for them. Valence electron of potassium has the higher principal quantum number.

Orbital angular momentum number () is the second quantum number and it signifies the type of orbital. It is always greater than or equal to zero.  There are four main types of orbital: s, p, d, and f. Each orbital can hold two electrons. In a given shell, there are one ‘s’ orbital, three ‘p’ orbitals, five ‘d’ orbitals, and seven ‘f’ orbitals. ‘l’ = 0 for ‘s’ orbitals, ‘l’ = 1 for ‘p’ orbitals, ‘l’ = 2 for ‘d’ orbitals, and ‘l’ = 3 for ‘f’ orbitals. In potassium, there is only one valence electron; therefore, there is only one electron in the fourth shell and it can fit into the ‘s’ orbital. In potassium ion, there are eight valence electrons; therefore, two electrons can be found in the ‘s’ orbital and the remaining six electrons can be found in the three ‘p’ orbitals. Not all valence electrons of potassium ion and potassium have different ‘l’ value. This is because valence electron of potassium and two of the valence electrons of potassium ion are found in the ‘s’ orbital ().

Electrons found in higher shell numbers have higher energy. Valence electron of potassium is found in the fourth shell; therefore, it will have a higher energy than any of the valence electrons of potassium ion.

Example Question #4 : Quantum Numbers

Consider the following descriptions of quantum numbers:

A: energy level within a subshell

B: shape of orbital

C: spin of electron

D: energy level

Which of the following is the correct pairing of quantum numbers (1st, 2nd, 3rd, and 4th) with the given descriptions?

Possible Answers:

A: 3rd, B: 4th, C: 2nd, D: 1st

A: 4th, B: 2nd, C: 3rd, D: 1st

A: 2nd, B: 3rd, C: 4th, D: 1st

A: 3rd, B: 2nd, C: 4th, D: 1st

Correct answer:

A: 3rd, B: 2nd, C: 4th, D: 1st

Explanation:

There are four quantum numbers. The first quantum, or principal quantum number, is designated by the letter ‘n’. It signifies the electron shell, or the energy level of the electron. The second quantum number, or orbital angular momentum quantum number, is designated by the letter ‘l’. It signifies the shape (or type) of the orbital. The third quantum number, or magnetic quantum number, is designated by . This signifies the energy level within a subshell. Each orbital can be located in different orientations in space. For example, each of the three ‘p’ orbitals are oriented differently in space and have different energy levels. The third quantum number describes this phenomenon. Finally, the fourth quantum number, or spin quantum number, is designated by  and describes the spin of the electron. An electron can spin clockwise or counterclockwise.  describes the direction of the spin. Since an electron can only rotate two ways,  can only be two values  or .

These four numbers together describe the potential location of an electron inside an atom. Note that no two electrons can have the same set of quantum numbers (meaning at least one of the four numbers will be different).

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