Q: Which of the following is the correct formula for calculating the percent abundance of an isotope?

$\text{Percent abundance} = \frac{\text{Number of atoms of isotope}}{\text{Total number of atoms}} \times 100$

Question 1

A sample of chlorine consists of two isotopes: $^{35}\mathrm{Cl}$ (75.78% abundance, atomic mass = 34.969 amu) and $^{37}\mathrm{Cl}$ (24.22% abundance, atomic mass = 36.966 amu). What is the average atomic mass of chlorine?

Accepted Answer

$35.45$ amu

Question 2

Which of the following electron configurations represents a neutral atom of sulfur ($Z=16$)?

Accepted Answer

$1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^4$

Question 3

Which of the following best describes why sodium chloride ($\mathrm{NaCl}$) is solid at room temperature?

Accepted Answer

It has strong ionic bonds between oppositely charged ions.

Question 4

What is the molarity of a solution prepared by dissolving $5.85$ g of $\mathrm{NaCl}$ (molar mass $= 58.5$ g/mol) in enough water to make $0.500$ L of solution?

Accepted Answer

$0.200\ \mathrm{M}$

Question 5

Which of the following molecules has a trigonal planar shape?

Accepted Answer

$\mathrm{BF}_3$

Question 6

Which of the following statements about resonance structures is correct?

Accepted Answer

Resonance structures represent different possible arrangements of electrons, not atoms.

Question 7

Which of the following is a polar molecule due to its molecular geometry?

Accepted Answer

$\mathrm{H}_2O$

Question 8

A gas sample has a pressure of $2.00$ atm, a volume of $4.00$ L, and a temperature of $300$ K. What will the volume be if the pressure is changed to $1.00$ atm and the temperature to $600$ K? (Assume ideal gas behavior.)

Accepted Answer

$16.0$ L

Question 9

Which of the following types of intermolecular forces is primarily responsible for the high boiling point of water?

Accepted Answer

Hydrogen bonding

Question 10

Which equation correctly relates the energy of a photon to its frequency?

Accepted Answer

$E = h 
u$

Question 11

Which of the following best explains why $\mathrm{CO}_2$ is nonpolar even though it contains polar bonds?

Accepted Answer

The molecule is linear, so the bond dipoles cancel each other out.

Question 12

Which of the following is the correct formula for calculating the percent abundance of an isotope?

Accepted Answer

$	ext{Percent abundance} = \frac{	ext{Number of atoms of isotope}}{	ext{Total number of atoms}} 	imes 100$

Question 13

Which of the following best describes the effect of increasing temperature on the average kinetic energy of gas molecules?

Accepted Answer

The average kinetic energy increases linearly with temperature (in Kelvin).

Question 14

Which of the following is the correct equation for the combined gas law?

Accepted Answer

$\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$

Question 15

Which of the following molecules has a tetrahedral electron geometry but a trigonal pyramidal molecular shape?

Accepted Answer

$\mathrm{NH}_3$

Question 16

Which of the following best explains why noble gases are chemically inert?

Accepted Answer

They have completely filled valence electron shells.

Question 17

Which of the following is the correct formula for calculating the energy of a photon given its wavelength $\lambda$?

Accepted Answer

$E = \frac{hc}{\lambda}$

Question 18

Which of the following best describes the effect of adding a nonvolatile solute to a solvent?

Accepted Answer

It lowers the vapor pressure of the solvent.

Question 19

Which of the following is the correct Lewis structure for carbon dioxide ($\mathrm{CO}_2$)?

Accepted Answer

O=C=O, with each O atom having two lone pairs

Question 20

Which of the following best explains why $\mathrm{H}_2O$ has a higher boiling point than $\mathrm{H}_2S$?

Accepted Answer

$\mathrm{H}_2O$ forms hydrogen bonds, while $\mathrm{H}_2S$ does not.

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