Exam 1
Identify the answer with the correct number of
significant figures:
(26.14/3.38) + 4.2 = ?
11.9
The answer obtained from the division is 7.7337 with
several more numbers after the decimal. In
multiplication and division, the number of significant
figures in the answer is limited by the number with the
fewest significant figures used to obtain it (3 sig. figs.).
When this answer is added to 4.2, the answer is
11.9337 and several more numbers after the decimal
point. For addition and subtraction, the number of
decimal places in the answer is limited by the number
with the fewest decimal places used to obtain it.
Because 4.2 only has one decimal place, the final
answer can only have one decimal place.
What is the electron geometry of an atom that is
sp3 hybridized?
tetrahedral
An atom that is sp3 hybridized is surrounded by four
regions of electrons. The electronic geometry is
tetrahedral.
In which of the following compounds does
chlorine have the highest (most positive) oxidation
number?
A. NaCl
B. ClO2-
C. HCl
D. Cl2
B
Elemental chlorine has an oxidation
number of zero. In both NaCl and HCl, the oxidation
number of chlorine is –1 (A, C). In ClO2–, the
oxidation number of chlorine is +3 (D).
Which gas at STP occupies the smallest volume?
A. 5.0 mol Kr
B. 5.0 mol HCl
C. 5.0 mol SO2
D. All occupy the same volume.
D
At standard temperature and pressure (STP), one mole
of any gas occupies 22.4 L (the identity of the gas does
not matter). Therefore, 5.0 moles of any gas will occupy
112 L (5 ́ 22.4).
J.J. Thomson is credited with the discovery of the
electron. What experimental technique was
employed?
A. Law of Multiple Proportions
B. Cathode Ray Tube
C. Gold foil experiment
D. Oil drop experiment
B
This just boils down to name recognition and
association with what each person is known for.
Dalton gave us the Law of Multiple Proportions,
Rutherford did the gold foil experiments, and Millikan
developed the oil drop experiment. This leaves
Thomson and his cathode ray tubes.
Rank the following compounds from lowest to
highest melting point.
Na2O
CO
MgO
CO < Na2O < MgO
ionic compounds have higher melting points than
covalent compounds, so CO will have the lowest
melting point. The other two compounds are ionic, and
their melting points are tied to their lattice energy.
Since MgO has Mg2+ and Na2O has Na+, MgO has a
higher lattice energy and a higher melting point.
Formic acid (HCO2H) is a weak acid. What is(are)
the spectator ion(s) in the acid-base neutralization
reaction of an aqueous solution of formic acid with
aqueous NaOH?
HCO2H (aq) + NaOH (aq) → HCO2Na (aq) + H2O (l)
Na+ is the only spectator
Because formic acid is a weak
acid, it does not completely dissociate into its ions in
solution. NaOH is a strong base, so it will completely
dissociate into its ions. All sodium salts are soluble in
water and strong electrolytes, so HCO2Na (sodium
formate) will dissociate completely into its ions. The
complete ionic equation can be written out:
HCO2H (aq) + Na+ (aq) + OH– (aq) →
HCO2– (aq) + Na+ (aq) + H2O (l)
Na+ is the only ion that appears on both sides of the
equation, so it is the only spectator ion in this reaction
(A, B, C, D, E).
What will happen to the volume of an ideal gas if
the temperature and pressure are both tripled, and
the number of gas particles is doubled?
A. It will stay the same
B. It will double
C. It will triple
D. It will increase by a factor of 4.5
E. It will increase by a factor of 18
B. It will Double
The ideal gas equation can be used to calculate the
volume after all of the changes. The variables need to
be defined:
3T (tripling the temperature), 3P (tripling the
pressure), 2n (doubling the moles).
A certain reaction produces 0.75 mol of Mn2O7.
What mass, in grams, is this?
166g
Identify the nonpolar molecules out of NF3, SO3, and PCl5.
SO3 and PCl5 only
All three of these molecules have polar bonds. NF3 is
polar since the three individual bond dipoles add up to
point straight down (A, D). The central atoms in SO3
and PCl5 do not have lone pair electrons, so the
bonding regions are symmetrical (trigonal planar and
trigonal bipyramidal, respectively). This allows the
individual bond dipoles to cancel each other out,
making each molecule nonpolar (B, C).
Identify the limiting reactant when 6.50 g O2 are
reacted with 8.75 g of N2 and 12.5 g H2O.
For NH4NO3 80.04 g/mol; O2 32.00 g/mol;
N2 28.02 g/mol; H2O 18.02 g/mol
O2 (g) + 2 N2 (g) + 4 H2O (l) → 2 NH4NO3 (s)
Nitrogen it the limiting reactant
What volume of 0.500 M hydrochloric acid solution
needs to be added to excess sodium carbonate in
order to cause the evolution of 14.5 L of carbon
dioxide gas at STP?
2 HCl (aq) + Na2CO3 (aq) →
2 NaCl (aq) + H2O (l) + CO2 (g)
𝟐.𝟓𝟗𝑳 𝐇𝐂𝐥
At standard temperature and pressure (STP), one mole
of any gas will occupy 22.4 liters. This conversion can
be used right away to determine the number of moles of
CO2 that will be produced. If the number of moles of
CO2 are known, the volume of HCl can be calculated
through stoichiometry.
The electron configuration [Kr] 4d10 corresponds
to which ion(s)?
A. Sn2+ only
B. Sn4+ only
C. Cd2+ only
D. Two of these ions
E. All three of these ions.
D
Neutral tin has the electron configuration of [Kr] 5s2
4d10 5p2. To form Sn2+, the two electrons in the 5p
orbital are lost, and the new electron configuration is
[Kr] 5s2 4d10 (A). If two more electrons are lost to
form Sn4+, they come from the 5s orbital, and the new
electron configuration is [Kr] 4d10 (B).
Neutral cadmium has the electron configuration of
[Kr] 5s2 4d10. To form Cd2+, the two electrons from the
5s orbital are lost, and the new configuration is
[Kr] 4d10 (C). Therefore, the correct answer is D.
Which of the following, as a pure substance,
exhibits the strongest intermolecular force?
Hint: Draw Lewis Structures!
carbon tetrafluoride, boron trifluoride, or sulfur tetrafluoride
sulfer tetrafluoride (SF4)
all three molecules have dispersion
forces, and none are capable of hydrogen bonding. All
three molecules have polar bonds, but they cancel
each other out in CF4 and PF5 due to the symmetrical
geometry. The lone pair electrons on sulfur in SF4
pushes the four bonding regions down. The four
individual dipoles can now add up to create an overall
dipole that points down. Because it is a polar molecule,
it has dipole-dipole interactions, and these are stronger
than dispersion forces (A, B).
Consider the decomposition reaction for
mercury (II) oxide, which occurs when the
compound is heated:
2 HgO (s) → 2 Hg (l) + O2 (g)
A 9.25 g sample of mercury (II) oxide
(216.6 g/mol) is heated to produce 3.75 g of
pure mercury (200.6 g/mol). What is the percent
yield for this reaction?
𝟒𝟑.𝟖% 𝒚𝒊𝒆𝒍𝒅
The given mass of pure mercury is the
actual yield of the reaction. The percent yield is
calculated by dividing the actual yield by the
theoretical yield. We need to calculate the theoretical
yield from the given mass of HgO.
Ammonium nitrate can decompose explosively
when heated according to the balanced equation:
2 NH4NO3 (s) → 2 N2 (g) + 4 H2O (g) + O2 (g)
How many liters of gas would be formed at 450 °C
and 2.00 atm pressure by an explosion initiated with
450 grams of NH4NO3?
𝟓𝟖𝟒 𝑳
Blue photons (475 nm) are incident on two metals
– Mg and K. Which metal(s) will experience the
photoelectric effect?
Threshold Energies:
Mg: 5.90 × 10−19 J
K: 3.69 × 10−19 J
K only
A compound has an empirical formula of C2H3Cl
and its molar mass is measured to be ~250 g/mol.
How many atoms of each element are present in
the molecular formula of this compound?
C8H12Cl4
The mass of the given formula is ~62 g/mol. The given
formula must be multiplied by four to get a molar mass
~248 g/mol. The molecular formula would be
C8H12Cl4.
Using Hess’s law, determine the ∆H of the first
reaction below given the data for the other two
reactions.
Zn(s) + 2 AgCl(s) → 2 Ag(s) + ZnCl2(s)
Zn(s) + Cl2(g) → ZnCl2(s) ∆H°f = –415.1 kJ/mol
Ag(s) + ½ Cl2(g) → AgCl(s) ∆H°f = –127.1 kJ/mol
-160.9 kJ
(Ch10, Obj #11) Solid zinc is a reactant in the main
reaction, and it is also a reactant in the first reaction
given. It does not need to be manipulated.
Zn (s) + Cl2 (g) → ZnCl2 (s) ∆H°f = –415.1 kJ/mol
AgCl is a reactant in the main reaction and a product
in the second reaction. This reaction needs to be
flipped, which changes the sign for ∆H. It also needs
to be doubled, which doubles its ∆H.
2 AgCl(s) → 2 Ag(s) + Cl2(g) ∆H°f = +254.2 kJ/mol
Now, the two equations should add up to the main
equation.
Zn (s) + Cl2 (g) → ZnCl2 (s) ∆H°f = –415.1 kJ/mol
2 AgCl(s) → 2 Ag(s) + Cl2(g) ∆H°f = +254.2 kJ/mol
A gas mixture containing NO2, CO2 and SO2
occupies 50.0 L at 1.75 atm and 325 K. If 37.7 g of
NO2 are in the mixture, what is the mole fraction
and the partial pressure of NO2 in this mixture?
Mole Fraction: 𝟎.𝟐𝟓𝟎
Partial Pressure: 𝟎.𝟒𝟑𝟕𝒂𝒕𝒎