Lewis Structures
Which species below contains a central atom with an expanded octet?
A. N3-
B. SF4
C. PF3
D. BF3
B!
Elements in the second row of the periodic table are not capable of expanding their octet (A, D). Phosphorous is in Group V, and in PF3 it has a complete octet of three single bonds and one lone pair of electrons (C). Sulfur is in Group VI, and in SF4 it has four single bonds and one lone pair of electrons.
Which of the following has the smallest bond
angle?
A. ammonia, NH3
B. methane, CH4
C. water, H2O
D. All three have the same bond angle.
C!
The central atom in each of NH3, CH4, and H2O is surrounded by four regions of electrons, and all three would have a tetrahedral electronic geometry. NH3 has one lone pair of electrons on nitrogen, and H2O has two lone pairs of electrons on oxygen. Lone pair electrons are held closer to the central atom, and as a result, cause the bond angle to shrink (CH4 = 109.5°, NH3 = ~107.5°, and H2O = ~104.5°).
Which compound has the strongest intermolecular forces?
A. CH3CH2CH3
B. CH3OCH3
C. CH3CH2OH
D. CH3CH2Cl
C!
The strength of the intermolecular forces are in the order hydrogen bonding > dipole-dipole > London dispersion. All four of these compounds have London dispersion forces, but B, C, and D have dipole-dipole forces that A does not (A). Of the three that remain, only C has hydrogen bonding (B, D). In order for a compound to have hydrogen bonding, it needs to have a hydrogen atom bonded to oxygen, nitrogen, or fluorine.
Which of the following structures has a central atom with sp2 hybridization?
A. O3
B. CH2O
C. HCN
D. both O3 and CH2O
E. both O3 and HCN
D!
four regions of electrons surrounding them (109.5° for carbon, ~ 107.5° for nitrogen, and ~ 104.5° for oxygen).
When a compound of calcium and nitrogen forms,
what is its chemical formula?
A. CaN
B. CaN3
C. Ca3N2
D. Ca2N3
E. Ca3N
C!
Calcium is a Group II metal, and it forms the common cation Ca2+. Nitrogen is a Group V nonmetal, and it forms the common anion N3–. To balance the charges, three Ca2+ cations combine with two N3– anions.
How many lone pairs of electrons are around the central atom in IF3?
A. 0
B. 1
C. 2
D. 3
E. 4
C!
IF3 has a total of 28 valence electrons (7 from each atom), and iodine is the central atom since it can
Which of these has a bent molecular shape?
NO2-, OCN-, or ClO2-
A. NO2 – only
B. OCN– only
C. ClO2 – only
D. Two of them
E. All three of them
D!
A bent molecular shape can be adopted for a molecule when the central atom is surrounded by either 3 or 4 regions of electrons (B, E). When three, the central atom has two bonding regions and one lone pair of electrons (C). When four, the central atom has two bonding regions and two lone pairs of electrons (A). In this series, both NO2 – and ClO2 – adopt a bent molecular shape
Which of the following compounds exhibits dipole-dipole intermolecular interactions as its strongest intermolecular force in the pure compound?
A. N2
B. CO2
C. NaCl
D. CH4
E. HCl
E!
Similar to Question #20. In order for a molecule to exhibit dipole-dipole interactions, it needs to have a net dipole. Both N2 and CH4 are nonpolar (A, D), and CO2 is nonpolar due to the linear geometry (B). NaCl is an ionic compound, and even though a dipole exists, ionic compounds have ion-dipole interactions, and these are stronger than dipole-dipole interactions (C). HCl is the only polar molecule in this list due to the large electronegativity difference between hydrogen and chlorine.
Consider nitrogen (N2) and diazine (N2H2).
A. Nitrogen contains more p-bonds than s-bonds
while diazine contains more s-bonds than pbonds.
B. Nitrogen contains more s-bonds than p-bonds
while diazine contains more p-bonds than sbonds.
C. Both molecules contain equal numbers of s
and p-bonds.
D. Both molecules contain more s-bonds than pbonds.
N2 has 10 total valence electrons, and N2H2 has 12. The triple bond in N2 is composed of one sigma bond and two pi bonds. The double bond in N2H2 is composed of one sigma bond and one pi bond. In total, N2 contains two pi bonds and one sigma bond; N2H2 contains one pi bond and three sigma bonds.
Which of the following polyatomic ions contains three oxygen atoms and has an overall charge of 2-?
A. nitrate
B. phosphate
C. carbonate
D. sulfate
E. perchlorate
F. ammonium
C!
In order, the formulas for these polyatomic ions are NO3 – , PO4 3– ,CO3 2– , SO4 2– , ClO4 – , and NH4 +.
Consider the molecules HCN, CO2 and SO3. Which are non-polar?
A. all three are non-polar
B. HCN and CO2
C. CO2 and SO3
D. HCN and SO3
E. only CO2 is non-polar
C!
All three contain polar bonds (CtoN; CtoO; StoO), but due to the geometry of CO2 and SO3, these individual dipoles cancel each other out, making each molecule nonpolar.
According to the VSEPR Model, which one of the following molecules has a tetrahedral molecular geometry?
A. NH3
B. CCl4
C. CO2
D. SO3
E. HCN
F. SF4
B!
A molecule with a tetrahedral molecular geometry has a central atom with four bonding regions of electrons and zero nonbonding regions of electrons. In terms of molecular geometry, NH3 is trigonal pyramidal (A), CO2 is linear (C), SO3 is trigonal planar (D), HCN is linear (E), and SF4 is square planar (F).
Choose the member from each set that has the higher boiling point. SO2 or CO2 NH3 or PH3
A. SO2 and NH3
B. SO2 and PH3
C. CO2 and NH3
D. CO2 and PH3
In the first pair, SO2 has London dispersion forces as well as dipole-dipole interactions; CO2 only has London dispersion forces (C, D). In the second pair, NH3 has hydrogen bonding, PH3 does not.
What hybridization is predicted for the central nitrogen atom in the azide polyatomic ion, N3– ?
A. SP3
B. SP3D
C. SP
D. SP3D2
E. SP2
C!
The Lewis structure for N3 – shows 16 valence electrons where the central nitrogen forms a double bond with each of the other two nitrogen atoms. All three nitrogen atoms have formal charges. Since the central nitrogen is surrounded by two regions of electrons, its hybridization would be sp
Determine the combined mass percent composition of oxygen and nitrogen in Urea, CH4N2O.
73.3%
Which of the following compounds can exhibit resonance?
A. NH4+
B. N2H4
C. ClO3-
D. ClF3
C!
Resonance is the ability for p-electrons to be delocalized. NH4 + and ClF3 do not have any p bonds (A, D). N2H4 has a p bond between the two nitrogen atoms but moving the electrons to one of the nitrogen atoms will cause the other nitrogen atom to violate the octet rule (B).
The H–C–C bond angle in C2H4 is:
120!
The two central carbon atoms in this structure are both surrounded by three regions of electrons. This means the geometry around them is trigonal planar, their bond angle is 120°, and they are both sp2 - hybridized.
Sodium cyanide (NaCN) and acetonitrile (CH3CN) both contain a polar covalent triple bond between a carbon and nitrogen atom, however, their physical properties are very different. For example, one has a melting point of −45 °C and the other 563 °C. Why are the melting points so different?
A. NaCN has the lower melting point because it
has a higher molar mass.
B. NaCN has the higher melting point because it
has a higher molar mass.
C. NaCN has the higher melting point because it
is ionic, whereas CH3CN is molecular.
D. NaCN has the higher melting point because it
is molecular, whereas CH3CN is ionic.
E. NaCN has the lower melting point because it is
ionic, whereas CH3CN is molecular.
C!
NaCN is an ionic compound composed of a metal and a polyatomic anion. Ionic compounds tend to have higher melting points than covalent compounds because they are able to form stable lattice structures.
The CO bond in COCl2 can be described as…
A. a σ bond and a π bond, both involving sp
hybrid orbitals on C.
B. a σ bond involving an sp hybrid orbital on C,
and a π bond involving a p orbital on C.
C. a σ bond and a π bond, both involving sp2
hybrid orbitals on C.
D. a σ bond involving an sp2 hybrid orbital on C,
and a π bond involving a p orbital on C.
E. two π bonds, both involving a p orbital on C.
F. a σ bond involving an sp3 hybrid orbital on C,
and a π bond involving a p orbital on C.
G. a σ bond and a π bond, both involving sp3
hybrid orbitals on C.
D!
This formula has a total of 24 valence electrons, and carbon is its central atom. Since the carbon is surrounded by three regions of electrons, it is sp2 hybridized. This hybrid orbital is used to form the sigma bond. The leftover p orbital is used to form the pi bond.
Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?
C3H5O2
Assuming a 100g sample, adipic acid will be composed of 49.32g carbon, 43.84g oxygen, and 6.85g hydrogen. Each of these needs to be converted to moles
Each value is then divided by the smallest number to obtain the initial formula C1.5H2.5O. Whole numbers can be obtained by multiplying by a factor of two to give C3H5O2.
The following statements compare hydrazine, N2H4, and diazene, N2H2. Which statement is correct?
A. The N—N bond of hydrazine is stronger and
shorter than that of diazene.
B. The N—N bond of hydrazine is weaker and
shorter than that of diazene.
C. The N—N bond of hydrazine is stronger and
longer than that of diazene.
D. The N—N bond of hydrazine is weaker and
longer than that of diazene
D!
The two nitrogen atoms in N2H4 are bonded using a single bond, while the two nitrogen atoms in N2H2 are bonded using a double bond. Bonds become shorter when more electrons are used (single > double > triple) and the shorter the bond, the stronger it is.
According to the VSEPR Model, which of the following molecules would have the smallest bond angle?
A. CO2
B. SO2
C. BeCl2
D. HCN
B!
The central atom in each of CO2, BeCl2, and HCN is surrounded by two regions of electrons, leading to a linear geometry and a 180° bond angle. The central atom in SO2 is surrounded by two bonding regions and one nonbonding region, leading to a bent geometry and a bond angle slightly less than 120° (A, C, D).
Which of the following would be expected to be the most soluble in benzene, C6H6, a non-polar solvent? A. CH3OH
B. CaCl2
C. HCN
D. CS2
D!
Since benzene is a non-polar solvent, it would be most suitable for dissolving non-polar compounds (like dissolves like). The compounds in A and C are both polar because they have permanent dipoles (A, C). CaCl2 is an ionic compound (ion-dipole, B). A bond between carbon and sulfur is not polar, so CS2 is a non-polar molecule.
The π bond in ethylene (C2H4) results from the overlap of:
A. sp3 hybrid orbitals
B. s atomic orbitals
C. sp hybrid orbitals
D. sp2 hybrid orbitals
E. p atomic orbitals
E!
The two carbons in ethylene are both sp2 hybridized, and these hybrid orbitals are used to form s bonds. The p bond is formed using the leftover p orbital.
Phosphorus is used in making some matches. When burned in oxygen the combustion product is 43.64 % phosphorus and 56.36 % oxygen. What is the molecular formula if the molar mass of the compound is 284 g/mol?
If we assume a 100g sample of this compound, we can convert the percentages directly to grams and calculate the number of moles for each.
Dividing each by 1.41 gives the mole ratios and an initial formula of P1O2.5. Each subscript is doubled to get to whole numbers and a formula of P2O5. This formula only has a mass of 142 g/mol; doubling each subscript again gives P4O10 and a formula mass of 284 g/mol.