CHEM 121A Review

What is the following number to four significant figures in standard exponential notation: -0.0357202?

-3.572 × 10^-2

Which will have the highest concentration of potassium ion: 0.20 M KCl, 0.15 M K2CrO4, or 0.080 M K3PO4?

0.15 M K2CrO4

What is the following number to four significant figures in standard exponential notation: 102.53070?

1.025 × 10^2

What is the correct number of protons, electrons, and neutrons in the isotope 36Cl, respectively?

17 protons, 17 electrons, 19 neutrons

Cations are larger than their corresponding neutral atoms.

False

Cl− is bigger than I−

False

F− is smaller than F.

False

True or false: The hydrogen atom is most stable when it has a full octet of electrons.

False An octet requires eight electrons in the valence shell. Hydrogen's valence shell is the n=1 shell, which can carry only two electrons at most. This is the 1s subshell, with one orbital.

A p orbital on one atom cannot make a bond to an s orbital on another atom.

False Compounds that include hydrogen and an element from the p block of the periodic table will include a bond that involves the s orbital of the hydrogen atom (the only one it has) and the p orbital of the other atom, such as HBr.

Part complete The 1s orbital has a nodal plane.

False The shape of the 1s orbital is spherical and does not include any nodal planes or regions in which the electron density is zero.

What would the drawing look like if the volume and temperature remained constant while you removed enough of the gas to decrease the pressure by a factor of 3?

It would contain a third of the molecules.

Lone pairs of electrons on an atom in a molecule influence the shape of a molecule.

True A lone electron pair is attracted to only a single nucleus, as opposed to a bonding pair, which is attracted to two nuclei. Because of this, the electron domain of the lone pair is larger and will exert a greater repulsive force on the adjacent electron domains in the molecule, compressing their bond angles.

In order to make a covalent bond, the orbitals on each atom in the bond must overlap.

True Covalent bonds are formed through the overlap of half-filled valence orbitals. Because of the overlapping, it is most probable that electrons are located within the region of the bond.

Part complete The 2p orbital has a nodal plane.

True The p orbitals are dumbbell-shaped with a nodal plane through the nucleus. This means that the probability of finding an electron at the point where the two lobes of a p orbital meet is zero.

What is the sign of q for this process?

q<0 The bicycle pump feels warm to the touch after compression because the system (the air and the pump) is giving off heat. This means that the system is losing heat, so q is negative.

Assuming the pump and the air in it comprise the system, what is the sign of w when you compress the air?

w>0 The sign of w is relative to the system. You are doing work on the system by compressing the air inside the pump, so w is positive.

If an atom has the electron configuration 1s22s22p4, how many electrons must it gain to achieve an octet?

2 electrons The atom in question has an electron configuration of 1s22s22p4. Of these orbitals, only the 2s and 2p are in the valence shell, and so only the electrons in these subshells will be considered. This means that there are 6 electrons in the valence shell. An octet requires eight electrons in the valence shell, so we can find the required number of electrons for an octet by subtracting the number of electrons in the valence shell from 8. Electrons needed for octet=8−(6 valence electrons)=2 electrons

How many electrons must a sulfur atom gain to achieve an octet in its valence shell?

2 electrons The condensed electron configuration of sulfur is [Ne]3s23p4. The valence shell is n=3 and includes all subshells. This means that there are six electrons in the valence shell of sulfur. To reach an octet, we will need two more electrons. number of electrons needed to reach an octet=8−6 valence electrons=2 electrons

What is the following number to four significant figures in standard exponential notation: 0.000257870?

2.579 × 10^-4

Which will contain the greater number of moles of potassium ion: 30.0 mL of 0.15 M K2CrO4 or 25.0 mL of 0.080 M K3PO4?

30.0 mL of 0.15 M K2CrO4

How many nitrogen atoms are in 10.2 g of copper(II) nitrate?

6.56 × 10^22 atoms

What is the following number to four significant figures in standard exponential notation: 656,980?

6.570 × 10^5

What is the following number to four significant figures in standard exponential notation: 0.008543210?

8.543 × 10^-3

Based on your answers to Parts A and B, can you determine the sign of ΔE for compressing the air in the pump?

The sign of ΔE cannot be absolutely determined. We do not know the exact quantities of the energy gained through the work (w) done on the system or the energy lost through heat (q) from the system. Since ΔE=q+w and w is positive while q is negative, it is not possible to determine from the available information if the system experienced a net loss or net gain of energy. If q is greater in magnitude than w, ΔE would be negative. However, if w is greater in magnitude than q, ΔE would be positive.

Given molecular dipole moments for pentene, ethyl ether, bromo-butane and butan-1-amine as 0.2 D, 1.4 D, 2.8 D, and 3.2 D respectively, which molecule is expected to have the highest boiling point?

butan-1-amine

Which quantum numbers are involved in the transition that requires the least energy?

n=1 to n=2 Looking at the figure to the left, the amount of energy is represented by the y-axis. The energy difference between n=1 and n=2 represents a transition that requires the least amount of energy.

As drawn, which quantum numbers are involved in the transition that requires the most energy?

n=1 to n=4 Looking at the figure to the left, the amount of energy is represented by the y-axis. The energy difference between n=1 and n=4 represents a transition that requires the greatest amount of energy.

Based on the drawing, put the following in order of increasing wavelength of the light absorbed during the transition.

n=2 to n=4, n=3 to n=1, n=3 to n=2, n=1 to n=2 Wavelength and energy are inversely proportional. Therefore, the smaller the change in energy between levels, the longer the wavelength that is emitted or absorbed.