Question

53) A buffer solution contains carbonic acid (H CO) and sodium bi carbonate (NaHCO), each at a concentration of 0.100 M. The relevant equilibrium is shown below. What is H:COs(ag)+ HOo (a)+HCOs (ag) K.-4.5x 10 54) At 40 C, the pH of water is 6.77, what is lon-Prodact Constant for Water, Kw.at 40°C 55) If a solution of acetic acid has a pH-2.74 and a pK474, wh in the solution? Show your calculations is the ratio of (CH Co/ICOH) 56) For the reaction 3H2(g) + N2(g)-2NH3(g) determne the equilibrium constant at 60 rcíf at this temperature, the equilibrium concentrations are: [N:]-4.53 M: [H]-2.49 M, and [NH-.62 57) When a sample of aqueous hydrochloric acid was neutralized with aqueous sodium calorimeter, the temperature of 100.0 g of water surrounding the reaction increased from 25.0°C to e secific heat of water is 1.00 cal (g"C) calculaethe quantity of energy in calories involved in neutralization reaction How many kJ of heat are needed to raise the temperature of 0.20 kg of water from 20 °C to 100 "C i preparing a cup of coffee? (Cwat 4.18 kJkg"C) 59) Consider thereversible reaction: A(g)-2B(g) At equilibrium, the concentration of A is 0.381 M and that of B is 0.154 M. What is the value of the equilibrium constant, Keq? 60) A candy sample of 1.00 g is completely combusted in a bomb calorimeter. The calorimeter contains 982 g of water, and the measured temperature increase is 2.62°C. What is the fuel value of the candy sample in nutritional Calories? [SH. 1.00 calg°C 61) When 50.0 mL of 0.200 MAgNOs and 50.0 mL of 0.200 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 23.2 °C to 24.8 °C. The temperature increase is caused by the following reaction AgNO (ag)+HC(ag)AgCI(s)+ HNOs(aq) Calculate the enthalpy change (AH) for this reaction in kJ/mol of AgNO). (Assume that the combined solution has a mass of 100.0 g and a specific heat of 4.18 J/g °C.) 62) A 40.0 mL sample of 0.100 M HCI is titrated with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of NaOH: (a) 0.0 mL, (b) 10.0 mL, (c) 20.0 mL, (d) 40.0 mL, (e) 60.0 mL. A plot of the pH of the solution as a function of the volume of added titrant is known as a pH titration curve. Using the available data points, plot the pH titration curve for the above titration. 63-80) Problems in the Homework Assignments for Chapters 7 and 8.

Answer 1

53. Given the K_a of carbonic acid (H₂CO₃) is K_a = 4.5*10^-7,

pK_a = -log (K_a)

= -log (4.5*10^-7)

= 6.347

Use the Henderson-Hasslebach equation to determine the pH.

pH = pK_a + log [NaHCO₃]/[H₂CO₃]

====> pH = 6.347 + log (0.1 M)/(0.1 M)

====> pH = 6.347 + log (1.0)

====> pH = 6.347 + 0.0 = 6.347

====> pH ≈ 6.3 (ans).

54. The pH of water is 6.77. Therefore,

pH = -log [H₃O⁺] = 6.77

====> log [H₃O⁺] = -6.77

====> [H₃O⁺] = antilog (-6.77)

====> [H₃O⁺] = 1.698*10^-7

The hydronium ion concentration in water is 1.698*10^-7 M.

Consider the auto-ionization of water as below.

2 H₂O (l) <======> H₃O⁺ (aq) + OH^- (aq)

Due to the 1:1 nature of ionization,

[H₃O⁺] = [OH^-] = 1.698*10^-7 M.

The ion product of water is given as

K_w = [H₃O⁺][OH^-]

= (1.698*10^-7 M)*(1.698*10^-7 M)

= 2.883*10^-14 M² ≈ 2.88*10^-14 M²

The ion product is expressed as a pure number; hence, the ion product of water at 40ºC is 2.88*10^-14 (ans).

55. Use the Henderson-Hasslebach equation.

pH = pK_a + log [CH₃CO₂^-]/[CH₃CO₂H]

Plug in values and get

2.74 = 4.74 + log [CH₃CO₂^-]/[CH₃CO₂H]

=====> -2.00 = log [CH₃CO₂^-]/[CH₃CO₂H]

=====> [CH₃CO₂^-]/[CH₃CO₂H] = antilog (-2.00)

=====> [CH₃CO₂^-]/[CH₃CO₂H] = 0.01 (ans).

56. The equilibrium constant for the reaction is given as

K = [NH₃]²/[N₂][H₂]³

= (7.62 M)²/(4.53 M)(2.49 M)³

= 0.830 M^-2

The equilibrium constant is expressed as a dimensionless quantity; hence, the equilibrium constant is 0.830 (ans).