Another significant exception to the octet rule involves those elements that tend to form
question
Which pair of atoms forms the most polar bond?
C and O
N and O
N and F
C and F
answer
The greater the electronegativity difference between the bonding atoms, the more polar the bond. F is the most electronegative of these elements and C is the least electronegative, therefore the bond between them is the most polar bond.
question
Which pair of atoms forms a nonpolar covalent bond?
C and O
Na and Cl
B and O
C and I
answer
A bond is nonpolar if the difference in electronegativity between the atoms that form the bond is less than 0.4. See the figures below. Both C and I have electronegativities of 2.5, so a C−I bond is nonpolar.
question
formal charge equation
answer
# of valence electrons -(lone pairs + # of bonds)
question
formal charge within a compound
answer
The formal charge of an atom in a Lewis structure is equal to its number of valence electrons (5) minus its number of nonbonding electrons (2) minus one-half times the number of its bonding electrons (6). 5−2−12(6)=0. This N atom has typical number of bonds for nitrogen, so the formal charge is expected to be zero.
question
Use formal charge to choose the best Lewis structure for CH3SOCH3.
answer
All carbon, oxygen , and sulfur atoms have octets. The Lewis structure has the correct total number of electrons (2(4)+6(1)+6+6=26). The Lewis structure does not have any formal charges on any of the atoms, including the sulfur atom (6−2−1/2(8)=0).
question
tetrahedral
answer
A tetrahedral geometry corresponds to four electron groups around the central atom. The central atom in C2H2 has less than four electron group. You may want to review
question
linear
answer
A linear geometry corresponds to two electron groups around the central atom.
question
seesaw
answer
-4 bonds, 1 lone pair
-The central S atom in SF4 has one lone pair. A seesaw geometry corresponds to five electron groups around the central atom, with one of the electron groups being a lone pair.
question
Predict the relative bond angles in CCl4 and H2O.
CCl4 bond angles > H2O bond angle
H2O bond angles > CCl4 bond angle
CCl4 bond angles = H2O bond angle
Relative bond angles cannot be predicted.
answer
Both central atoms have four electron groups, but two of the electron groups in H2O are lone pairs. Lone pairs take up more angular space than bonding pairs, so the bond angle is smaller in H2O than in CCl4.
question
Predict the molecular geometry about N in the molecule CH3NHCH3.
Predict the molecular geometry about in the molecule
bent
linear
trigonal planar
trigonal pyramidal
answer
There are four electron groups around the central atom in CH3NHCH3 and one of these electron groups is a lone pair. This corresponds to a trigonal pyramidal geometry, which is a common geometry for molecules that feature nitrogen as their central atom.
question
Which molecule is polar?
CO2
BF3
PCl5
SF2
answer
SF2 has a bent geometry because it has four electron groups around the central atom and two of those electron groups are lone pairs. The polar bonds do not cancel each other in this geometry, resulting in a polar molecule.
question
A chemical system produces 170 kJ of heat and does 20. kJ of work. What is ΔE for the surroundings?
150 kJ
190 kJ
-190 kJ
-150 kJ
answer
When a system gives off heat, qsys is negative, and when a system does work, wsys is also negative.
ΔEsys===qsys+wsys−170kJ+(−20.kJ)−190kJ
and, so,
ΔEsurroundings==−ΔEsystem+190kJ
question
Which sample is most likely to undergo the smallest change in temperature upon the absorption of 100 kJ of heat?
57 g lead
5 g lead
57 g water
5 g water
answer
The total change in temperature depends on the both the mass of the sample and its specific heat capacity. The greater the mass, the less its temperature changes upon absorption of a fixed amount of heat. The larger the specific heat capacity, the less its temperature changes upon absorption of a fixed amount of heat. For water Cs=4.18J/g⋅∘C and for lead Cs=0.128J/g⋅∘C; so for an equal mass of each the water has a smaller temperature change than lead.
question
A cylinder with a moving piston expands from an initial volume of 0.400 L against an external pressure of 2.10 atm. The expansion does 254 J of work on the surroundings. What is the final volume of the cylinder?
1.59 L
1.19 L
0.794 L
121 L
answer
To determine the correct answer, use the equation w=−PΔV where V=Vf−Vi. Use the conversion factor 101.3J =1L⋅atm to convert between J and L⋅atm.
w=−P(Vf−Vi)
−254J×1L⋅atm101.3J=2.10atm(Vf−0.400L)
Rearrange and solve for Vf to get Vf=1.59L.
question
Hydrogen gas reacts with oxygen to form water.
2H2(g)+O2(g)→2H2O(g)ΔH=−483.5kJ
Determine the minimum mass of hydrogen gas required to produce 176 kJ of heat.
1.47 g
11.1 g
0.734 g
0.728
answer
Determine the number of moles of hydrogen from the amount of heat required and be careful with units.
176kJ×2molH2483.5kJ×2.016gH21molH2=1.47gH2
question
What is the main goal of chemistry?
answer
The main goal of chemistry is to seek to understand the behavior of matter by studying the behavior of atoms and molecules.
question
What are two different ways to classify matter?
answer
by state and by composition
question
How do solids, liquids, and gases differ?
answer
a solid has a fixed volume and rigid shape; liquids assume the shape of their container; gases always assume the shape and volume of their container
question
Potassium has three naturally occurring isotopes with masses of 38.96 amu, 40.96 amu, and 39.96 amu and natural abundances of 93.30 %, 6.70 %, and 1.2×10−2 %, respectively. Calculate the atomic mass of potassium.
answer
Once you convert the percent abundance values to fraction or decimal form, you can calculate the atomic mass, which is a weighted average (weighted according to natural abundance), as follows:
atomic mass of potassium=(0.9330)38.96 amu+(6.70×10−2)40.96 amu+(1.2×10−4)39.96 amu
question
A pure silver ring contains 1.60×1022 silver atoms. How many moles of silver atoms does it contain?
answer
amount = 2.7×10−2 mol Ag
Avogadro's number, 6.022×1023 particles/mol , is used to convert between the number of moles and the number of atoms, molecules, particles, etc. The number of silver atoms divided by Avogadro's number yields the number of moles of silver.
question
What is the difference between an element and a compound?
answer
An element is a pure substance which cannot be decomposed into simpler substances. A compound is composed of two or more elements in fixed proportions.
question
Calculate the amount of copper (in moles) in a 47.9 g pure copper sheet.
answer
0.754 mol Cu
Atomic mass is used to convert between the mass and number of moles of an element. The mass of copper in grams divided by its atomic mass (63.55 g/mol) yields the number of moles of copper. Note that the units of grams cancel during calculation, and the units of moles appear in the numerator.
question
What is the difference between a homogeneous and a heterogeneous mixture?
answer
A homogeneous mixture has the same composition throughout, while a heterogeneous mixture has different compositions in different regions
question
How many carbon atoms are there in a 2.2-carat diamond? Diamonds are a form of pure carbon. (1 carat = 0.20 grams)
answer
Once you've determined the mass of carbon in grams, the atomic mass is used to convert the mass to the number of moles, and Avogadro's number is used to calculate the number of atoms. The calculation using the appropriate conversion factors is summarized as follows:
2.2 carat×0.2 g1 carat×1 mol C12.01 g×6.022×1023 carbon atoms1 mol C
When converting the mass in grams to the number of moles, you should use the atomic mass in g/mol as a conversion factor. Set up the equation such that like units cancel, leaving you with mol in the numerator.
question
1 mol
answer
6.02214×1023 particles
Avogadro's number
question
A titanium cube contains 2.85 ×1023 atoms. The density of titanium is 4.50 g/cm3. Find the edge length of the cube.
answer
The number of atoms can be used to determine the volume of the cube using the appropriate conversion factors. From the number of titanium atoms, we can find the number of moles of titanium using Avogadro's number. Then, the atomic mass is used to convert the number of moles to the mass of titanium in grams, which is then converted to volume using density.
The conversion factors are set up such that all undesired units cancel. The edge length (l) of the cube is then calculated by taking the cubic root of the volume: l=V1/3=(5.05 cm3)1/3.
1.71 cm
question
What observations did Antoine Lavoisier make?
answer
He noticed that there was no change in the total mass of material within the container during combustion
question
What law did he formulate did Antoine Lavoisier make
answer
the law of conservation of mass
question
What theory did John Dalton formulate?
answer
atomic theory
question
State the law of definite proportions.
answer
All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements.
question
Why do we use other bonding theories in addition to the Lewis model?
answer
Lewis model cannot predict all of the properties of a molecule or ion.
question
Draw the Lewis structure for HCN.
answer
The most accurate Lewis structure of HCN, which has 10 valence electrons, allows for each atom to have a complete octet and the smallest formal charge. Every atom in this structure has a formal charge of 0. Note that the triple bond contains a total of six electrons shared between carbon and nitrogen, so only three of those electrons are used to calculate the formal charge of carbon or nitrogen.
question
Hibridization of HCN that has linear geometry
answer
sp
question
Predict the molecular geometry about the carbon atom.
answer
linear, Given that carbon has no lone pairs of electrons, the molecular geometry is identical to the electron geometry.
question
According to Coulomb's law, if the separation between two particles of the same charge is doubled, the potential energy of the two particles:
answer
is one-half as high as it was before the separation bc According to Coulomb's law the potential energy of two charged particles is proportional to the product of the two charges divided by the distance between them. Since the distance is doubled, the potential energy is divided by two.
question
Which electron in fluorine is most shielded from nuclear charge?
An electron in the 1s orbital
An electron in the 2s orbital
An electron in a 2p orbital
none of the above (All of these electrons are equally shielded from nuclear charge.)
answer
An electron in a 2p orbital
-Valence electrons are more shielded from nuclear charge than core electrons. The valence electrons in F are the n=2 electrons, or the a 2p electrons.
question
Choose the correct electron configuration for S.
3s23p4
1s22s22p63s23p4
1s22s22p4
1s22s22p63p4
answer
1s22s22p63s23p4
question
Which set of four quantum numbers corresponds to an electron in a 3d orbital?
n=3,l=0,ml=4,ms=−1/2
n=3,l=1,ml=3,ms=1/2
n=3,l=3,ml=0,ms=−1/2
n=3,l=2,ml=1,ms=1/2
answer
n=3,l=2,ml=1,ms=12
The 4 in 3d indicates that n=3. Since this is a d orbital, l is 2. The possible values of ml for l=2 are −2, −1, 0, 1, and 2, so ml=1 is allowed. Lastly, ms can be either 1/2 or −1/2.
question
Which element has the smallest atomic radius?
Si
Be
C
F
answer
F
Atomic radii decrease from left to right across a row in the periodic table, so Be is the largest and F is the smallest
question
Which statement is true about effective nuclear charge?
answer
Effective nuclear charge decreases as you move to the left across a row in the periodic table.
question
Which species is diamagnetic?
Co2+
N3−
Fe2+
Ti
answer
N3−
Recall that a diamagnetic atom or ion is one in which all electrons are paired. For N3−, all of the orbitals that contain electrons are completely filled ([He]2s22p6).
question
When 0.630 gMn is combined with enough hydrochloric acid to make 100.0 mL of solution in a coffee-cup calorimeter, all of the Mn reacts, raising the temperature of the solution from 23.4 ∘C to 28.4 ∘C.
Find ΔHrxn for the reaction as written. (Assume that the specific heat capacity of the solution is 4.18 J/g∘C and the density is 1.00 g/mL.)
answer
-182 kJ
question
Which statement is true of the temperature of the two substances when they reach thermal equilibrium? (Assume no heat loss other than the thermal transfer between the substances.)
answer
The final temperature of both substances will be closer to the initial temperature of substance B than the initial temperature of substance A. The higher the heat capacity of a substance, the less its temperature changes upon the exchange of a fixed amount of heat.
question
The standard enthalpy of formation for ethanol [CH3COOH(l)] is −484.3 kJ/mol. What is the correct formation equation corresponding to this ΔHof?
answer
2C(s,graphite)+2H2(g)+O2(g)→CH3COOH(l)
The standard enthalpy of formation is the change in enthalpy that occurs when one mole of the compound forms from its constituent elements in their standard states. Carbon is graphite, and hydrogen and oxygen are diatomic gases.
question
Which set of compounds is arranged in order of increasing magnitude of lattice energy?
Which set of compounds is arranged in order of increasing magnitude of lattice energy?
CsI < CaS < KCl
CsI < KCl < CaS
KCl < CsI < CaS
CaS < KCl < CsI
answer
CsI < KCl < CaS
The magnitude of the lattice energy increases with increasing ionic charge and decreases with increasing ionic radius. CaS has the highest magnitude lattice energy because the charges in CaS are twice the magnitude for each ion compared to KCl and CsI. KCl has a higher magnitude lattice energy because the ionic radii of K+ and Cl− are smaller than for Cs+ and I−.
question
What is the empirical formula of the compound with the molecular formula C12H8?
answer
C3H2
The subscripts in the molecular formula must equal n times the subscripts in the empirical formula (where n is an integer). Both 12 and 8 are divisible by 4, so the empirical formula is C3H2.
question
Which substance is an ionic compound?
SrI2
N2O4
He
BH3
answer
Strontium iodide is composed of one metal and one nonmetal and is therefore an ionic compound.
question
What is the correct formula for the compound that forms between calcium and sulfur?
CaS3
CaS2
Ca2S
CaS
answer
CaS
The charges of the ions in the compound must sum to 0. The charge on Ca is 2+ and the charge on S is 2−, and there are one of each of these, so the charges in this formula sum to 0.
question
Name the compound BaI2
barium(II) diiodide
barium diiodide
barium iodide
barium(II) iodide
answer
barium iodide
Name the cation, barium , followed by the anion, iod, with the − ide ending. The charges of group 1A and group 2A are invariant from one compound to another and therefore do not require Roman numerals to specify their charges.
question
Name the compound Cr(C2H3O2)2.
chromium(II) acetate
chromium bicarbonate
chromium diacetate
chromium(II) carbonate
answer
chromium(II) acetate
Cr(C2H3O2)2 is an ionic compound, containing both a metal and a polyatomic anion. The acetate ion is C2H3O2−. The charge on the Cr (chromium) is 2+ to balance the charge of the two acetate anions. A Roman numeral (II) is necessary because Cr can have multiple charges. The cation is named chromium(II), followed by the name of the anion, acetate.
question
Name the compound P2Br4.
phosphorus dibromide
diphosphorus tetrabromide
phosphorus(II) bromide
phosphorus bromide
answer
diphosphorus tetrabromide
P2Br4 is composed of two nonmetals and is therefore a molecular compound. The names of molecular compounds contain prefixes to indicate the number of each type of atom (di for two atoms and tetra for four atoms). They do not contain Roman numerals. The name of the compound starts with the first prefix (di), the first element (phosphorus), the second prefix (tetra), and the second atom, changing the ending ( bromide ).
question
how to find molecular formula
answer
1. find the whole number multiple by → molecular weight / empirical formula weight
2. multiply each of the subscripts by this multiple
question
how to find empirical formula
answer
- mass in 100g of compound
- ratio of atoms
- simplest ratio of atoms
- empirical formula
question
Which gas sample has the greatest volume at STP?
Which gas sample has the greatest volume at STP?
50.0 g Cl2
50.0 g F2
50.0 g O2
None of the above (They all have the same volume.)
answer
50.0 g O2
The volume of a gas at a fixed temperature and pressure depends only the number of moles of gas present. Therefore, the sample with the greatest number of moles has the greatest volume. Since the masses of the three options are equal, the gas with the lightest molar mass has the greatest number of moles.
molCl2(71g/mol)
question
Which gas has the greatest kinetic energy at STP?
Which gas has the greatest kinetic energy at STP?
He
Ne
Ar
None of the above (All have the same kinetic energy.)
answer
None of the above (All have the same kinetic energy.)
The kinetic energy of a gas depends only on its temperature, so all of these atoms have the same kinetic energy.
question
The kinetic energy of a gas depends only on its temperature, so all of these atoms have the same kinetic energy.
Kr < C3H8 < N2 < CH4
CH4 < N2 < Kr < C3H8
CH4 < C3H8 < N2 < Kr
CH4 < N2 < C3H8 < Kr
answer
CH4 < N2 < C3H8 < Kr
The density of a gas at STP is proportional to its molar mass.
CH4 (16g/mol) < N2 (28g/mol) < C3H8 (44g/mol) < Kr (84g/mol)
question
How is the miscibility of a liquid with water related to the liquid's polarity?
answer
The more polar the liquid, the more likely that it is miscible with water.
question
What is the ion-dipole force? Why is it important?
answer
The ion-dipole force occurs when an ionic compound is mixed with a polar compound and is especially important in aqueous solutions of ionic compounds
question
What is hydrogen bonding? How can you predict the presence of hydrogen bonding in a compound?
answer
If hydrogen is bonded to O, F, or N
The large electronegativity difference between hydrogen and these electronegative elements means that the H atoms will have fairly large partial positive charges (delta+), while the F, O, or N atoms will have fairly large partial negative charges (delta-)
question
What is surface tension? How does surface tension result from intermolecular forces? How is it related to the strength of intermolecular forces?
answer
Surface tension is the tendency of liquids to minimize their surface area. Molecules at the surface have relatively fewer neighbors with which to interact, because there are no molecules above the surface. Consequently, molecules at the surface are inherently less stable — they have higher potential energy — than those in the interior. In order to increase the surface area of the liquid, some molecules from the interior have to be moved to the surface, a process requiring energy. The surface tension of a liquid is the energy required to increase the surface area by a unit amount. Surface tension decreases with decreasing intermolecular forces.
question
What is viscosity?
answer
Viscosity is the resistance of a liquid to flow. Viscosity is measured in a unit called the poise (P), defined as 1 g/cm·s
question
Choose the substance with the highest boiling point.
NaF
CS2
HF
CH4
F2
answer
NaF
question
What is the strongest type of intermolecular force present in O2?
answer
dispersion
question
Which of the following compounds has dipole-dipole interactions as the strongest intermolecular force?
CH3Br
CBr4
BrCH2CH2OH
Br2
answer
CH3Br
question
Choose the substance with the lowest vapor pressure at a given temperature.
CO2
BF3
PF5
Ar
CaI2
answer
CaI2
question
Solid potassium chlorate (KClO3) decomposes into potassium chloride and oxygen gas when heated. How many moles of oxygen form when 58.5 g completely decomposes?
answer
Write a balanced chemical equation:
2KClO3(s)⟶Δ2KCl(s)+3O2(g)
Then convert the mass of KClO3 to moles of KClO3 before using the stoichiometric conversion factors to convert to moles of O2.
58.5gKClO3×1molKClO3122.55gKClO3×3molO22molKClO3=0.716molO2
question
Identify the correct balanced equation for the combustion of pentene (C5H10).
Identify the correct balanced equation for the combustion of pentene .
C5H10(g)→5H2(g)+5C(s)
2C5H10(g)+15O2(g)→10H2O(g)+10CO2(g)
2C5H10(g)+10O2(g)→10H2O(g)+10CO2(g)
2C5H10(g)+15O2(g)→10H2CO3(g)
answer
2C5H10(g)+15O2(g)→10H2O(g)+10CO2(g)
The products of combustion for a hydrocarbon, such as pentene, are carbon dioxide and water. The set of coefficients (2, 15, 10, 10) in this reaction results in the same number of each type of atom on both sides of the equation. There are ten C atoms, twenty H atoms, and thirty O atoms on each side of the equation.
question
If 51.4 g S is allowed to react as completely as possible with 106.1 g F2(g), what mass of the excess reactant is left?
If 51.4 is allowed to react as completely as possible with 106.1 , what mass of the excess reactant is left?
36.5 g S
45.2 g F2
21.6 g S
16.0 g S
answer
Determine the limiting reactant by converting the mass of F2 to mass of S (converting mass of F2 to moles F2; then to moles F2; and, finally, to mass of S). Compare the mass of S needed with the mass of S available.
106.1gF2×1molF238.0gF2×1molSi3molF2×32.06gS1molS=29.8gSneeded
The mass of excess S is 51.4gS−29.8gS=21.6gS.
question
What is the molarity of a solution containing 59.4 g of MgCl2 dissolved in 1.00 L of solution?
What is the molarity of a solution containing 59.4 of dissolved in 1.00 of solution?
1.60 M
0.624 M
59.4 M
0.594 M
answer
Molarity is moles of solute divided by liters of solution.
59.4gMgCl2×1molMgCl295.21gMgCl2=0.62388molMgCl2
molarity=molsoluteLsolution=0.62388molMgCl21.00L=0.624MMgCl2
question
What mass (in grams) of Mg(NO3)2 is present in 104 mL of a 0.400 M solution of Mg(NO3)2?
What mass (in grams) of is present in 104 of a 0.400 solution of ?
3.59 g
6.17 g
0.0416 g
38.6 g
Which solution will form a precipitate when mixed with a solution of aqueous Ba(NO3)2?
Which solution will form a precipitate when mixed with a solution of aqueous ?
NH4Cl(aq)
BaCl2(aq)
MnCl2
K2CO3(aq)
answer
Only insoluble substances form precipitates. KNO3 is soluble and BaCO3 is insoluble. The precipitate is BaCO3.
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