On Ramps Chemistry Midterm 1 Review

The sequence of numbers: 1, 0.25, 0.11, 0.0625, 0.04, 0.0277 has what functional relationship to n = 1, 2, … and is associated with what important physical phenomenon?

• 1/n Coulombic forces

• n² particle in a box energy levels

• 1/n² electron energy transitions

• ln n absolute entropy

Answer explanation

You see that the values fall off quickly and sticking in values of n = 1, 2, … you see that 1/n2 is the function that describes the sequence values. Of course there turns out to be a physical significance. Balmer found out that there was a 1/n² relationship for the spacing of the hydrogen emission lines in the visible region. Rydberg came up with his equation with the Rydberg constant and then later it was found that this constant precisely matched Schodinger’s equation prediction for a hydrogen electron transitioning through the n quantum number.

If an electron falls from the n = 4 level to the energy level n=3 level, which of the following statements is true about the radiation emitted?

• The lowest energy emission in the infrared region has occurred

• The highest energy emission in the infrared region has occurred

• The lowest energy emission in the microwave region has occurred

• The highest energy emission in the microwave region has occurred

Answer explanation

This question deals with the atomic spectra in which scientists generated emission spectra for hydrogen gas and observed not the continuum predicted by classical mechanics, but rather discrete lines. There is only one equation used to elucidate the answer: that being Rydberg’s equaton that states- v (frequency) = R(1/n₁² - 1/n₂²). Knowing that the IR region results from an electron falling to the n=3 level rules out answers C and D. Knowing that sticking values of n = 3 and 4 produces the smallest value for the Rydberg equation means that the lowest energy emission is observed.

Tags

NGSS.HS-PS4-3

If an electron has an approximate wavelength of 1 nm, estimate the wavelength of a DNA strand that weighs 10^6 g/mole and traveling at the same velocity.

• 10^-15 m

• 1 m

• 10^-18 m

• 10^-35 m

Answer explanation

A proton weighs about 1000 times more than an electron, and the DNA strand is 106 more massive than the proton. So altogether, there is a nine order of magnitude larger mass for the DNS compared to the electron, so it has a 10-18 m wavelength.

Tags

NGSS.HS-PS4-1

What is the uncertainty in Dr. Laude’s position if he weighs 220 pounds (this is about 100 kg) and ambles down the street at 2 meters per second with a 1% uncertainty in his velocity?

• 3 x 10^-36 m

• 6 x 10^-19 m

• 6 x 10^20 m

• 2 m

Answer explanation

This is a classic plug and chug, but even without doing the calculation, you know that the correct answer is A just from looking an the uncertainty principle equation. On one side you have the number (1 x 10^-34)/2. On the other side you have pretty sizeable numbers like 100 kg and an uncertainty in velocity of 0.02 m/s. So the correct answer is going to be really close in value to 10^-34 and only one answer is anywhere close.

What is Dr. Laude’s wavelength if he weighs 220 pounds (fiction) and walks down the street at 2 meters per second? (The good news is that you will always be given the equations you need as well as the constants. Note that everything is in mks units except the Dr. Laude’s weight, but it happens that there are 2.2 pounds in a kilogram.)

• 2.5 x 10^-35 m

• 6 x 10^-19 m

• 6 x 10^20 m

• 2 m

Tags

NGSS.HS-PS4-1

Using the wave functions provided below, which of the following would be the most likely location for a particle in a 1 nm length box that is excited to the n = 2 quantum level?

• 500 pm

• 0 pm

• 1 nm

• 750 pm

Answer explanation

To begin, the wave functions are not an indication of where a particle is. It is the wave function squared. So when we look at the n = 2 energy level, we have to square the sine wave and we see that not only the spacing ¼ of 1 nm is a maximum, but also the spacing 3/4th of one nm, which is the most negative amplitude at 750 pm, is now the only one of the answers that is a non-zero value (0 , 500 and 1000 pm are all zero likelihood.)

Using the wave functions provided below, which of the following would be the most likely location for a particle in a 1 nm length box that is excited to the n = 2 quantum level?

• 500 pm

• 0 pm

• 1 nm

• 750 pm

What are the number of potential energy terms in the particle in a box and hydrogen atom calculations?

• 0,1

• 1,1

• 1,0

• 0,0

Answer explanation

There are four protons and three electrons in Be+ . Each of the three electrons is attracted to the positive nucleus (the number of protons doesn’t matter, it is the number of electrons.) As for repulsion, the first electrons is repulsed by electrons 2 and 3. The second electron is repulsed by electron 3. That makes three total repulsions. All the other repulsions you can identify are redundant.

Which of the following quantum number values could apply to the valence electrons of platinum?

• n = 5, l = 4, m₁ = 3, mₛ = -1/2

• n = 5, l = 5, m₁ = 3, mₛ = 1/2

• n = 5, l = 2, m₁ = 2, mₛ = -1/2

• n = 6, l = 2, m₁ = 2, mₛ = 1/2

Answer explanation

The correct answer is C. In order to solve this problem, you must first find where platinum is on the periodic table. Platinum valence electrons are in a 5d orbital, so it’s n value is 5 and it’s l value is 2 (which tells you it is a d orbital). Once you know you have a 5d orbital for the valence, the other values can vary from ml = -2, -1, 0, 1, 2 and an electron spin that is always either +1/2 or -1/2.

Tags

NGSS.HS-PS1-1

What is the total number of electrons that could be described by the quantum numbers n = 5 and m₁ = 3?

• 4 electrons

• 25 electrons

• 5 electrons

• 10 electrons

• 50 electrons

Answer explanation

The correct answer is A. To solve this problem, you must find out which l values have an ml value of 3. The possible l values for n = 5 that can also have ml = 3 are only l = 3 and l = 4. So now we have two orbitals that fit those boundary conditions. Each orbital can hold a maximum of two electrons, so the total number of electrons that could be described are 4 (2 x 2 = 4).