AP CHEM BIG IDEA #2 MULTIPLE CHOICE

How many moles of Na₂SO₄ must be added to 500 millimeters of water to create a solution that has a 2-molar concentration of the Na⁺ ion? (Assume the volume of the solution does not change)

.5moles (Moles Na=1 we need to add .5)

An evacuated rigid container is filled with exactly 2.00g of hydrogen and 10.00g of neon. The temperature of the gases is held at 0°C and the pressure inside the container is a constant 1.0ATM. What is the mole fraction of neon in the container?

0.33 (Hydrogen is diatomic. Moles H₂ (1) + Moles Ne (.5), total moles (1.5) → .5/1.5=.33)

A mixture of helium and neon gases has a total pressure of 1.2 atm. If the mixture contains twice as many moles of helium as neon, what is the partial pressure due to neon?

0.4atm (Dalton's law, the partial pressure of a gas depends on the number of moles of the gas that are present -- (1/3)(1.2)=.4atm)

What is the bond order for the O-O bond in ozone, O₃?

1.5

A substance is dissolved in water, forming a 0.50-molar solution. If 4.0 liters of solution contains 240 grams of the substance, what is the molecular mass of the substance?

120g/mol

The six carbon atoms in a benzene molecule are shown in different resonance forms as three single bonds and three double bonds. If the length of a single carbon-carbon bond is 154 pm and the length of a double carbon-carbon bond is 133 pm, what length would be expected for the carbon-carbon bonds in benzene?

140 pm (Resonance is used to describe a situation that lies between single and double bonds, so the bond length would also be expected in between that of single and double bonds.)

An evacuated rigid container is filled with exactly 2.00g of hydrogen and 10.00g of neon. The temperature of the gases is held at 0°C and the pressure inside the container is a constant 1.0ATM. What is the volume of the container?

33.5L (PV=nRT - at STP 1 mol takes up 22.4: 1.5m*22.4=33.5)

Which of the following expressions is equal to the density of helium gas at standard temperature and pressure?

4/22.4 g/l (Density is measured in grams per liter. helium = 4)

An ideal gas fills a balloon at a temperature of 27°C and 1 atm pressure. By what factor will the volume of the balloon change if the gas in the balloon is heated to 127°C at a constant pressure?

4/3 (Ideal gas law -- V₁/T₁=V₂/T₂)

A mixture of gases contains 1.5 moles of oxygen, 3.0 moles of nitrogen, and .5 moles of water vapor. If the total pressure is 700 mmHg, what is the partial pressure of the nitrogen gas?

420mmHg (Dalton's law, the partial pressure of a gas depends on the number of moles of the gas that are present -- total moles 1.5+3+.5=5.0 - 3molesN, then (3/5.0)(700mmHg)=420mmHg)

Nitrogen gas was collected over water at 25°C. If the vapor pressure of water at 25°C is 23mmHg, and the total pressure in the container is measured at 781mmHg, what is the partial pressure of nitrogen gas?

758mmHg (Dalton's law, the partial pressure of a gas depends on the number of moles of the gas that are present--781-23=758mmHg)

The following three substances are kept in identical containers 25°C. All three substances are liquid. Ethanol, acetone, and ethylene glycol. Which substance would have the highest vapor pressure?

Acetone because it exhibits the weakest intermolecular forces (Vapor pressure arises from molecules breaking free from the intermolecular forces holding them together. Acetone having no hydrogen bonding has the weakest intermolecular force therefore the highest vapor pressure.)

The following three substances are kept in identical containers 25°C. All three substances are liquid. Ethanol, acetone, and ethylene glycol. Which of the substances would be soluble in water?

All three substances would be soluble in water due t their permanent dipoles (Water is very polar, any substance with polar molecules would be soluble in water. As all three molecules are polar, all three liquids would be soluble in water)

*Which of the following pairs of elements is most likely to create an interstitial alloy?

Aluminum and lead (*Interstitial alloys form when atoms of greatly different sizes combine. Aluminum is small--fitting in between the lead atoms, lead is large)

*A 22.0 g sample of an unknown gas occupies 11.2 liters at standard temperature and pressure. Which of the following could be the identity of the gas?

CO₂ (*Molecular weight -- moles = (L)/(L/m))

Why does CaF₂ have a higher melting point than NH₃?

CaF₂ is an ionic substance and it requires a lot of energy to break up an ionic lattice (Ca is a metal, F is a nonmetal. Their electro-negativities differ sufficiently for them to create an ionic bond, which is stronger than all the other bond types--except for network covalent bonding)

The following three substances are kept in identical containers 25°C. All three substances are liquid. Ethanol, acetone, and ethylene glycol. Which substance would have the highest boiling point?

Ethylene Glycol because it has the most hydrogen bonding (Hydrogen bonds are the strongest types of intermolecular forces when dealing with molecules of the same size, so Ethylene having twice as many hydrogen bonds takes the cake)

*Which of the molecules listed below has the largest dipole moment?

HCl (*HCl is held by a covalent bond and larger polarity--nonpolar molecules have dipole moments of zero)

An evacuated rigid container is filled with exactly 2.00g of hydrogen and 10.00g of neon. The temperature of the gases is held at 0°C and the pressure inside the container is a constant 1.0ATM. Which gas particles have a higher RMS velocity and why?

Hydrogen because it has a lower molar mass (The gas molecules have the same amount of kinetic energy due to their temperature being the same, meaning only the molecule with less mass will have a higher velocity)

*Which of the following lists of species is in order of increasing boiling points?

H₂,N₂,NH₃ (*Both H and N experience london dispersion forces but since N is larger it has a higher boiling point. NH₃ is polar and undergoes hydrogen bonding, so it has the strongest intermolecular interaction and the highest boiling point)

A liquid whose molecules are held together by only which of the following forces would be expected to have the lowest boiling point?

London dispersion forces (liquid with a low boiling point must be held together by weak intermolecular forces, of which london dispersion forces are the weakest kind)

*Which of the following compounds would have the highest lattice energy?

MgCl₂ (*MgCl₂ is smaller meaning it will have a higher lattice energy, as atomic radius is inversely proportional with bond energy)

An evacuated rigid container is filled with exactly 2.00g of hydrogen and 10.00g of neon. The temperature of the gases is held at 0°C and the pressure inside the container is a constant 1.0ATM. Which gas has the higher boiling point and why?

Neon because it has more electrons (Both gases only have london dispersion forces. The more electrons a gas has the more polarizable it is and the stronger intermolecular forces are.)

Why can a molecule with the structure NBr₅ not exist?

Nitrogen only has two energy levels and is thus unable to expand its octet (Only atoms with at least three energy levels (n = 3+) have empty d-orbitals that additional electrons can fit into, thus expanding their octet.

Which of the following molecules would exhibit the strongest dipole moment?

OF₂ (The other three options are non-polar, OF₂ has a bent shape that allows for dipole moments)

*Which of the following molecules will have a lewis dot structure with exactly one unshared electron pair on the central atom?

PH₃ (* Only PH₃ has a single unshared electron pair on its central atom)

*Which of the following species does not have a tetrahedral structure?

SF₄ (*SF₄ has 34 valence electrons distributed in the lewis dot structure with a non-tetrahedral shape)

A gas sample with a mass of 10 grams occupies 6.0 liters and exerts a pressure of 2.0 atm at a temperature of 26°C. Which of the following expressions is equal to the molecular mass of the gas? The gas constant, R, is 0.08 [(L)(atm)/(mol)(K)].

(10)(.08)(299)/(2.0)(6.0) (PV=nRT--solve for n)

In an experiment 2 moles of H₂ (g) and 1 mole of O₂ (g) were completely reacted, according to the following equations in a sealed container of constant volume and temperature: 2H₂(g) + O₂(g) → 2H₂O(g) If the initial pressure in the container before the reaction is denoted as P₁, which of the following expressions gives the final pressure, assuming ideal gas behavior?

(2/3)P₁