Kaplan MCAT General Chemistry Review

6. An electron returns from an excited state to its ground state, emitting a photon at = 500 nm. What would be the magnitude of the energy change if this process were repeated such that a mole of these photons were emitted?

A. 3.98 × 10^-19 J

B. 3.98 × 10^-21 J

C. 2.39 × 10⁵ J

D. 2.39 × 10³ J

7. Suppose an electron falls from n = 4 to its ground state, n = 1. Which of the following effects is most likely?

A. A photon is absorbed.

B. A photon is emitted.

C. The electron gains velocity.

D. The electron loses velocity.

8. Which of the following compounds is not a possible isotope of carbon?

A. ⁶C

B. ¹²C

C. ¹³C

D. ¹⁴C

9. According to the Heisenberg uncertainty principle, which of the following properties of a particle can an observer measure simultaneously?

I. Position

II. Momentum

III. Velocity

A. I and II

B. I and III

C. II and III

D. I, II, and III

10. Which of the following electronic transitions would result in the greatest gain in energy for a single hydrogen electron, assuming that its ground state is n = 1?

A. An electron moves from n = 6 to n = 2.

B. An electron moves from n = 2 to n = 6.

C. An electron moves from n = 3 to n = 4.

D. An electron moves from n = 4 to n = 3.

11. Suppose that a chemical species fills its orbitals as shown.

This compound could be said to obey which of the following laws of atomic physics?

A. Hund’s rule

B. Heisenberg uncertainty principle

C. Bohr’s model

D. Pauli exclusion principle

12. How many total electrons are in a ¹³³Cs cation?

A. 54

B. 55

C. 78

D. 133

13. The atomic mass of hydrogen is 1.008 amu. What is the percent composition of hydrogen by isotope, assuming that hydrogen’s only isotopes are ¹H and ²D?

A. 92% H, 8% D

B. 99.2% H, 0.8% D

C. 99.92% H, 0.08% D

D. 99.992% H, 0.008% D

14. Consider the two sets of quantum numbers shown in the table, which describe two different electrons in the same atom.

Which of the following terms best describes these two electrons?

A. Parallel

B. Opposite

C. Antiparallel

D. Paired

15. Which of the following species is represented by the electron configuration 1s²2s²2p⁶3s²3p⁶4s¹3d⁵ ?

A. Cr

B. Mn⁺

C. Fe²⁺

D. both (A) and (B)

16. Which of the following statements is NOT true of an electron’s ground state?

A. The electron is at its lowest possible energy level.

B. The electron is in a quantized energy level.

C. The electron is traveling along its smallest possible orbital radius.

D. The electron is static.

17. Which of the following experimental conditions would NOT excite an electron out of the ground state?

A. Radiation

B. High temperature

C. High pressure

D. None of the above

Small Group Questions

1. Which orbital fills first: 4s or 3d ? Which is first to give up electrons?

2. Why do atoms have neutrons? What purpose do they serve?

Explanations to Practice Questions

1. A

Because an electron was pulled off the neutral parent atom, consider how that parent atom was formed and which electron it would be willing to give up. Zn° has 30 electrons, so it would have an electron configuration of 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰. Subshells strongly prefer to be completely empty, exactly half full, or completely full. Therefore, the best way to lose two electrons and form Zn²⁺ is to pull them both out of the 4s-orbital. (B) implies that electrons are pulled out of the d-orbital, (C) presents the configuration of the uncharged zinc atom, and (D) shows the configuration that would exist if four electrons were removed.

2. B

The azimuthal quantum number l cannot be higher than n–1, ruling out (A). The m_l number, which describes the chemical’s magnetic properties, can only be an integer value between –l and l, and it cannot be equal to 1 if l = 0, ruling out (C) and (D).

3. C

The correct answer is (C). For any value of n there will be a maximum of 2n² electrons, i.e., two per orbital.

4. B

This formula describes the number of electrons in terms of the azimuthal quantum number l, which ranges from 0 to n–1, n being the principal quantum number.

Subshell Azimuthal Quantum Number (l) Number of Electrons s 02p 16d 210f 314

Using the data in the table, the quickest way to solve this problem is to choose a subshell and plug in its l value to solve for the number of electrons. If more than one option works, discard the others and test the remaining possibilities using another subshell.

5. D