Chem Exam #2 Flashcard Set 2

How many particles are in one mole?

6.022 x 10^23 particles

How do you convert grams to moles?

moles = grams / molar mass (g/mol)

What is the molar mass of H2O?

2(1.008) + 16.00 = 18.02 g/mol

How many moles are in 36.0 g of H2O?

36.0 g / 18.02 g/mol = 2.00 mol

How do you convert a Celsius temperature to kelvin?

K = °C + 273.15 (example: 25 °C = 298.15 K)

When can you use Celsius instead of kelvin in a gas problem?

Only for temperature DIFFERENCES (ΔT), since a 1 °C change equals a 1 K change. Absolute temperatures must be in K.

How do you convert mL to L and g to kg?

Divide by 1000 in both cases

What are the pressure unit equivalents for 1 atm?

1 atm = 760 torr = 760 mmHg = 101,325 Pa

How many particles does NaCl produce when dissolved?

2 (Na+ and Cl-)

How many particles does CaBr2 produce when dissolved?

3 (Ca2+ and 2 Br-)

How many particles does Al(NO3)3 produce when dissolved?

4 (Al3+ and 3 NO3-)

How many particles does glucose produce when dissolved?

1. Molecular compounds do not split into ions.

Is CCl4 polar or nonpolar, and why?

Nonpolar. It is symmetric, so its polar bonds cancel out.

Are hydrocarbons such as C6H14, C8H18, and C6H6 polar?

No, they are nonpolar

Name three polar molecules

H2O, NH3, and CH2F2 (also HCl and CH3OH)

What is the difference between intramolecular and intermolecular forces?

Intramolecular forces are the covalent bonds inside a molecule (strong). Intermolecular forces are attractions between molecules (much weaker) that determine whether a substance is a gas, liquid, or solid.

What two factors make an intermolecular attraction stronger?

Bigger charges and shorter distances

List the IMFs from weakest to strongest

Dispersion < dipole-dipole < hydrogen bonding < ion-dipole < ion-ion

What causes dispersion forces?

Constantly shifting electrons create brief temporary dipoles that attract neighboring molecules

Which molecules have dispersion forces?

ALL molecules and atoms

Why do bigger molecules have stronger dispersion forces?

Bigger electron clouds are more polarizable (easier to distort), producing stronger temporary dipoles

Which molecules have dipole-dipole forces?

Polar molecules (permanent positive and negative ends)

What is required for hydrogen bonding?

H bonded DIRECTLY to N, O, or F

Does an H bonded to carbon count for hydrogen bonding?

No. The H must be bonded directly to N, O, or F.

When do ion-dipole forces occur?

In mixtures of an ionic compound and a polar molecule, such as NaCl dissolved in water

What are ion-ion forces and what describes their strength?

The attraction between cations and anions in an ionic solid. Their strength is described by the lattice energy.

What are the three questions for identifying the IMFs in a molecule?

1) Is it polar? (yes = dipole-dipole, no = dispersion only) 2) Is H bonded to N, O, or F? (yes = hydrogen bonding) 3) Always add dispersion.

What IMFs does CH2F2 have?

Dispersion and dipole-dipole. It is polar but the H atoms are on carbon, so there is no hydrogen bonding.

What IMFs do H2O, NH3, CH3OH, and N2H4 have?

All three: dispersion, dipole-dipole, and hydrogen bonding (hydrogen bonding is the strongest)

What IMFs do CCl4, CO2, and C6H14 have?

Dispersion only (they are nonpolar)

What IMFs does HCl have?

Dispersion and dipole-dipole. Cl is not N, O, or F, so no hydrogen bonding.

Can pure CH3OCH3 (dimethyl ether) hydrogen-bond with itself?

No. It has no H bonded to N, O, or F (no O-H), so it has no H to donate.

What is the strongest IMF in pure H2O?

Hydrogen bonding

What is the strongest IMF between NaCl and water in a solution?

Ion-dipole forces

How does stronger IMF strength affect vapor pressure?

Lowers it (molecules escape the liquid less easily)

How does stronger IMF strength affect boiling point?

Raises it

What is the boiling point of a liquid?

The temperature at which its vapor pressure equals the external pressure

How does stronger IMF strength affect viscosity?

Raises it. Viscosity is a liquid's resistance to flow.

How does stronger IMF strength affect surface tension?

Raises it. Surface tension is the tendency of a liquid to minimize its surface area.

How does stronger IMF strength affect melting point and hardness of solids?

Both increase

How does stronger IMF strength affect the solubility of a solid?

Lowers it, because the lattice is harder to break apart

What are cohesive and adhesive forces?

Cohesive = attractions between liquid molecules. Adhesive = attractions between the liquid and the tube wall.

When does a liquid climb a narrow tube (capillary action)?

When adhesive forces are stronger than cohesive forces

Rank ethanol, dimethyl ether, and propane from lowest to highest vapor pressure

Ethanol < dimethyl ether < propane (ethanol H-bonds, dimethyl ether is polar, propane is nonpolar; stronger IMFs = lower vapor pressure)

What is the order of steps for ranking substances by IMF strength?

First look for hydrogen bonding, then polarity, then molecular size

What does "like dissolves like" mean?

Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Polar and nonpolar generally do not mix.

Will ethanol and water mix?

Yes, both are polar (miscible)

Will C8H18 (octane) and H2O mix?

No, nonpolar and polar liquids are immiscible

Will KI dissolve in water?

Yes, because of ion-dipole attractions with water

Will NaCl dissolve in benzene (C6H6)?

No. Benzene is nonpolar and cannot stabilize ions.

What do miscible and immiscible mean?

Miscible = the liquids mix. Immiscible = they do not mix.

What is lattice energy?

The energy released when gaseous ions come together to form an ionic solid. It is always negative; the size of the value shows bonding strength.

What does Coulomb's law say about lattice energy?

E is proportional to (q1 x q2) / r: larger charges and smaller distances give stronger attraction

What are the two rules for ranking the magnitude of lattice energy?

1) Bigger ion charges win. 2) If charges are equal, smaller ions win (they get closer together).

Rank NaCl, LiF, and MgO by increasing lattice energy magnitude

NaCl < LiF < MgO (LiF has smaller ions than NaCl; MgO has 2+/2- charges)

Which has the larger lattice energy, KBr or LiCl?

LiCl. The charges are equal and Li+ and Cl- are smaller.

What four properties describe a gas?

Pressure (P), volume (V), temperature (T in K), and amount (n in moles)

What is gas pressure?

The force per unit area exerted by gas particles hitting the walls of their container

Which elements are diatomic?

H2, N2, O2, F2, Cl2, Br2, I2

Are the noble gases diatomic?

No, He, Ne, Ar, Kr, and Xe exist as single atoms

What is the ideal gas law?

PV = nRT

What is the value of R for gas problems with atm and L?

R = 0.08206 L·atm/(mol·K)

When do you use R = 8.314 J/(mol·K)?

For energy and speed problems, such as u_rms

State Boyle's law

At constant T and n, pressure and volume are inversely related: P1V1 = P2V2

State Charles's law

At constant P and n, volume and temperature (in K) are directly related: V1/T1 = V2/T2

State Gay-Lussac's law

At constant V and n, pressure and temperature (in K) are directly related: P1/T1 = P2/T2

State Avogadro's law

At constant P and T, volume and moles are directly related: V1/n1 = V2/n2

What is the combined gas law?

P1V1/(n1T1) = P2V2/(n2T2). Cross out any variable that does not change.

Boyle's law: 30.0 L at 7.00 atm is compressed to 15.0 L. What is the new pressure?

P2 = (7.00)(30.0)/15.0 = 14.0 atm

Charles's law: 11.2 L at 15.0 °C is heated to 35.0 °C. What is the new volume?

V2 = 11.2 x (308.15/288.15) = about 12.0 L

A balloon is 28.4 L at 1.02 atm and 295.15 K. It rises to 0.350 atm and 228.15 K. What is the new volume?

V2 = 28.4 x (1.02/0.350) x (228.15/295.15) = about 64.0 L

A 9.25 L tire holds 0.612 mol of gas at 2.08 atm. What is the temperature?

T = PV/(nR) = (2.08)(9.25)/((0.612)(0.08206)) = about 383 K

What happens to the pressure if you squeeze a gas into half the volume (constant T and n)?

The pressure doubles (Boyle's law)

What is STP and what is the molar volume of an ideal gas at STP?

STP = 0 °C (273.15 K) and 1 atm. One mole occupies 22.414 L.

What is the formula for molar concentration of a gas?

C = n/V = P/(RT)

What is the formula for the density of a gas?

d = PM/(RT), with M in g/mol

Calculate the density of Cl2 (70.90 g/mol) at 1.10 atm and 318 K

d = (1.10)(70.90)/((0.08206)(318)) = about 2.99 g/L

Why are heavier gases denser at the same T and P?

Density is proportional to molar mass (d = PM/RT)

State Dalton's law of partial pressures

P_total = P_A + P_B + … (the total pressure is the sum of the partial pressures)

What is mole fraction and how does it connect to partial pressure?

chi_A = n_A / n_total, and P_A = chi_A x P_total

O2 has a mole fraction of 0.126 and a partial pressure of 0.45 atm. What is the total pressure?

P_total = 0.45 / 0.126 = about 3.6 atm

Why must you subtract water vapor pressure when a gas is collected over water?

The collected gas is mixed with water vapor, so P_total = P_gas + P_H2O. Only P_gas goes in PV = nRT.

What does the vapor pressure of water depend on?

Only temperature

H2 collected over water: total 751 torr, water vapor 19 torr, 0.755 L, 294 K. How many moles of H2?

P_H2 = 732 torr = 0.963 atm. n = PV/RT = (0.963)(0.755)/((0.08206)(294)) = about 0.0301 mol

What does kinetic molecular theory (KMT) assume about particle volume?

The particles themselves take up essentially no volume

What does KMT say about average kinetic energy?

It is proportional to temperature (in K) and does not depend on the type of gas

What does KMT assume about collisions?

They are elastic (no energy is lost)

What does KMT assume about particle interactions?

Particles do not attract or repel each other

At the same temperature, which moves faster: a light or a heavy gas?

The lighter gas, since all gases have the same average kinetic energy

Do all molecules in a gas sample have the same kinetic energy?

No. There is a spread of energies, so some molecules move faster than average.

What is the formula for root mean square speed?

u_rms = sqrt(3RT/M)

What units do you use in the u_rms formula?

R = 8.314 J/(mol·K), M in kg/mol, T in K

Calculate u_rms of N2 (0.02802 kg/mol) at 298 K

sqrt(3 x 8.314 x 298 / 0.02802) = about 515 m/s

How does u_rms change with temperature and molar mass?

Higher T = higher speed. Higher molar mass = lower speed.

What is mean free path?

The average distance a molecule travels between collisions. It gets shorter as pressure increases.

What is the difference between diffusion and effusion?

Diffusion = gas spreading from high to low concentration. Effusion = gas escaping through a tiny hole.

State Graham's law

rate1/rate2 = sqrt(M2/M1). Lighter gases effuse faster.

If He takes 42.0 s to effuse, how long does the same amount of CH4 take?

42.0 x sqrt(16.04/4.003) = about 84.1 s. The heavier gas takes longer.

What is a common mistake with Graham's law?

Flipping it. Time is the opposite of rate, so the heavier gas takes LONGER.

Under what conditions do gases behave most ideally?

High temperature and low pressure