Ch. 11
Calculate the molality of each of the following solutions:
(a) 0.710 kg of sodium carbonate (washing soda), Na2CO3, in 10.0 kg of water—a saturated solution at 0°C
(b) 125 g of NH4NO3 in 275 g of water—a mixture used to make an instant ice pack
a) 0.670 m
b) 5.68 m
Convert the values of Kc to values of KP or the values of KP to values of Kc.
a) Na2SO4·10H2O(𝑠)⇌Na2SO4(𝑠)+10H2O(𝑔) 𝐾𝑃=4.08×10−25 at 25°C
b) H2O(𝑙)⇌H2O(𝑔) 𝐾𝑃=0.122 at 50°C
Kc=Kp(RT)^-(delta N)
a) Kc = 5.31 × 10−39
b) Kc = 4.60 × 10−3
Calculate the pH and the pOH of each of the following solutions at 25 °C for which the substances ionize completely:
(a) 0.000259 M HClO4
(b) 0.21 M NaOH
(a) pH = 3.587; pOH = 10.413
(b) pOH = 0.68; pH = 13.32
Indicate the most important type of intermolecular attraction responsible for solvation in each of the following solutions:
(a) methanol, CH3OH, dissolved in ethanol, C2H5OH
(b) methane, CH4, dissolved in benzene, C6H6
(c) the polar halocarbon CF2Cl2 dissolved in the polar halocarbon CF2ClCFCl2
(d) O2(l) in N2(l)
(a) hydrogen bonds; (b) dispersion forces; (c) dipole-dipole attractions; (d) dispersion forces
Water gas, a mixture of H2 and CO, is an important industrial fuel produced by the reaction of steam with red hot coke, essentially pure carbon.
(a) Write the expression for the equilibrium constant for the reversible reaction
C(𝑠)+H2O(𝑔)⇌CO(𝑔)+H2(𝑔)Δ𝐻=131.30kJ
(b) What will happen to the concentration of each reactant and product at equilibrium if more C is added?
(c) What will happen to the concentration of each reactant and product at equilibrium if H2O is removed?
(d) What will happen to the concentration of each reactant and product at equilibrium if CO is added?
(e) What will happen to the concentration of each reactant and product at equilibrium if the temperature of the system is increased?
(a) 𝐾𝑐=[CO][H2][H2O]
(b) [H2O] no change, [CO] no change, [H2] no change
(c) [H2O] decreases, [CO] decreases, [H2] decreases
(d) [H2O] increases, [CO] increases, [H2] decreases
(e) [H2O] decreases, [CO] increases, [H2] increases.
Ammonia is a weak base that reacts with water according to this equation:
NH3(𝑎𝑞)+H2O(𝑙)⇌NH4+(𝑎𝑞)+OH−(𝑎𝑞)
Will any of the following increase the percent of ammonia that is converted to the ammonium ion in water?
(a) Addition of NaOH
(b) Addition of HCl
(c) Addition of NH4Cl
Only the addition of HCl, as protons (reactants) will shift the reaction to the right, and increase the percent of NH4+ in water.
Heat is released when some solutions form; heat is absorbed when other solutions form. Provide a molecular explanation for the difference between these two types of spontaneous processes in terms of bonds broken and bonds formed.
Heat is released when the total intermolecular forces (IMFs) between the solute and solvent molecules are stronger than the total IMFs in the pure solute and in the pure solvent: Breaking weaker IMFs and forming stronger IMFs releases heat. Heat is absorbed when the total IMFs in the solution are weaker than the total of those in the pure solute and in the pure solvent: Breaking stronger IMFs and forming weaker IMFs absorbs heat.
The following reaction has KP = 4.50 × 10−5 at 720 K.
N2(𝑔)+3H2(𝑔)⇌2NH3(𝑔)
If a reaction vessel is filled with each gas to the partial pressures listed, in which direction will it shift to reach equilibrium? P(NH3) = 93 atm, P(N2) = 48 atm, and P(H2) = 52 atm
The system will shift toward the reactants (left) to reach equilibrium.
The equilibrium constant (Kc) for this reaction is 5.0 at a given temperature.
CO(𝑔)+H2O(𝑔)⇌CO2(𝑔)+H2(𝑔)
On analysis, an equilibrium mixture of the substances present at the given temperature was found to contain 0.20 mol of CO, 0.30 mol of water vapor, and 0.90 mol of H2 in a liter. How many moles of CO2 were there in the equilibrium mixture?
0.33 moles of CO2 in the equilibrium mixture.
At 0 °C and 1.00 atm, as much as 0.70 g of O2 can dissolve in 1 L of water.
C=kp,
Answer: 2.80 grams
What is the value of the equilibrium constant at 500 °C for the formation of NH3 according to the following equation?
N2(𝑔)+3H2(𝑔)⇌2NH3(𝑔)
An equilibrium mixture of NH3(g), H2(g), and N2(g) at 500 °C was found to contain 1.35 M H2, 1.15 M N2, and 4.12 × 10−1 M NH3.
Kc= 6 x 10-2
The ionization constant for water (Kw) is 9.311 × 10−14 at 60 °C. Calculate [H3O+], [OH−], pH, and pOH for pure water at 60 °C.
[H3O+] = [OH−]= 3.051 × 10−7 M
pH = −log 3.051 × 10−7 = −(−6.5156) = 6.5156;
pOH = pH = 6.5156
Assuming ideal solution behavior, how many liters of HCl gas, measured at 30.0 °C and 745 torr, are required to prepare 1.25 L of a 3.20-M solution of hydrochloric acid? Assume 760 torr/atm.
1) Calculate moles of HCl (4.00 moles of HCl)
2) Convert Units for gas law (Temperature --> 303.15 K) & (Pressure --> 745 torr/760 torr/atm= 0.9803 atm
3) Calculate volume of HCl Gas using a rearranged version of the ideal gas law
Answer: 102 L
Suggest four ways in which the concentration of hydrazine, N2H4, could be increased in an equilibrium described by the following equation:
N2(𝑔)+ 2 H2(𝑔)⇌N2H4 (𝑔) Δ𝐻= 95 kJ
Explain why the ionization constant, Ka, for H2SO4 is larger than the ionization constant for H2SO3.
The ionization constant for sulfuric acid is higher than sulfurous acid because H2SO3 has more oxygen atoms, resulting in a higher oxidation state for sulfur +6 vs +4, respectively. This increased oxidation state increases the polarity of the O-H bonds, allowing for easier, more complete dissociation, and increasing the Ka for H2SO4.