University College - Chemistry Chem 112A Lecture Notes - Lecture 9: Buffer Solution, Ph

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harlequinimpala978

13 Dec 2016

School

WUSTL

Department

University College - Chemistry

Course

University College - Chemistry Chem 112A

Professor

Bleeke John

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[ ]0(cid:4667: buffers, conjugate acid-base equilibria, hno2(aq) + h2o(l) h3o+ (aq) + no2 (aq), ka = 4. 6 x 10 -4. 0. 25 x: ka =[ h3o+][ no2, using successive approximation, x = 4. 6 x 10-4, -log[x] = 3. 34, here ph = pka. H2o(l) (aq) (aq) + hno2(aq), kb = 2. 2 x 10-11. 0. 25 + x (aq) + h2o(l) oh- +x x: kb = x(0. 25+x)/(0. 25-x), using successive approximations x = 2. 2 x 10-11, -log[x] By adding the 0. 01 m strong acid, it gets consumer by 0. 01 m of a-, adding. Ka = x(0. 24+x)/(0. 26-x), using successive approximations x = 5. 0 x 10-4, gives us a ph of 3. 30. Notice ph hasn"t changed much, the solution is very slightly more acidic: add 0. 01 m strong base (assume no volume change) and we make the initial assumption that added base is consumed by reaction with ha.

Related Questions

1)Calculate the pH of a solution that results from mixing 10 mL of 0.11 M ammonia with 39 mL of 0.11 M ammonium chloride. The K_b value for NH₃ is 1.8 x 10^-5

2)A buffer that contains 0.18 M of an acid, HA and 0.31 M of its conjugate base A^-, has a pH of 4.66. What is the pH after 0.034 mol of HCl are added to 0.6 L of the solution

3)Calculate the pH during the titration of 30.00 mL of 0.1000 M NaOH(aq) with 0.1000 M HNO₃(aq) after 12 mL of the acid have been added

4)Calculate the pH during the titration of 20.00 mL of 0.1000 M HBrO(aq) with 0.1000 M LiOH(aq) after 8 mL of the base have been added. K_a of hypobromous acid = 2.3 x 10^-9.

5)Calculate the pH during the titration of 20.00 mL of 0.1000 M methylamine, (CH₃)NH₂(aq), with 0.2000 M HCl(aq) after 7.5 mL of the acid have been added. K_b of methylamine = 3.6 x 10^-4

6)Calculate the pH during the titration of 30.00 mL of 0.1000 M dimethylamine, (CH₃)₂NH(aq), with 0.1000 M HBr(aq) after 29 mL of the acid have been added. K_b of dimethylamine = 5.4 x 10^-4

⁷⁾Determine the volume in mL of 0.2 M HCl(aq) needed to reach the equivalence (stoichiometric) point in the titration of 49 mL of 0.25 M NH₂NH₂(aq)(aq). The K_b of hydrazine is 1.7 x 10^-6

⁸⁾Determine the pH at the equivalence (stoichiometric) point in the titration of 23 mL of 0.12 M C₆H₅COOH(aq) with 0.22 M NaOH(aq). The K_a of benzoic acid is 6.5 x 10^-5

Ignoring activities, determine the molar solubility of copper (I) azide (CuN₃) in a solution with a pH of 3.696. K_sp (CuN₃) = 4.9x10^{-9 ;} K_a (HN₃₎= 2.2x10^-5.

List of steps my teacher suggests we follow:

1. Write the pertinent reactions.

2. Write the charge balance equation.

3. Write the mass balance equations.

4. Write the equilibrium constant expressions for each reaction.

5. Count the equations and unknowns.

6. Solve for all unknowns.

How far I've gotten:

1. Pertinent reactions:

CuN₃ (s) <-> Cu⁺ (aq) + N₃^- (aq) K_sp= 4.9x10^-9

N₃^- (aq) + H₂O (l) <-> HN₃ (aq) + OH^- (aq) K_b = (K_w/K_a) = (1.0x10^-14/2.2x10^-5) = 4.55 x 10^-10

H₂O (l) <-> H⁺ (aq) + OH^- (aq) K_w = 1.0x10^-14

2. Write the charge balance equation:

Because the pH is fixed with no info given regarding the buffer used to fix the pH, there is no charge balance equation for this problem.

3. Write the mass balance equation(s):

[Total Cu⁺] = [Total N₃^-]

[Cu⁺] = [N₃^-] +[HN₃]

4. Write the equilibrium constant expressions for each reaction:

K_sp = 4.9x10^-9 = [Cu⁺] [N₃^-]

K_b = 4.55x10^-10 = ([HN₃] [OH^-] / [N₃^-])

K_w = 1.0x10^-14 = [H⁺] [OH^-]

Now use the charge balance, mass balance, and equilibrium constant expressions to solve for [Cu⁺], which is the molar solubility of CuN_3.

I tried to finish it 5 times and haven't been able to make any progress.

Show all your steps for calculation questions in order to get full points.

1. (4 pts) The solubility product (K_sp) of the insoluble salt AgBr is 5.0 x 10^-13. Calculate the equilibrium constant for the following reaction: Ag⁺(aq) + Br^-(aq) AgBr(s).

2. (6 pts) A solution is prepared by adding 50.0 mL of 0.050 M HCl to 150.0 mL of 0.10 M HNO₃.

a. Calculate the pH of the solution.

b. Calculate the concentrations of all species in this solution.

3. (4 pts) Calculate the volume of concentrated (12 M) HCl needed to prepare 1-L of a pH = 1.50 solution?

4. (5 pts) Arrange the following 0.10 M solutions in order of most acidic to most basic.

KOH, KCl, KCN, NH₄Cl, HCl

5. (6 pts) Calculate the pH of an aqueous solution containing 1.0 x 10^-2 M HCl and 1.0 x 10^-2 M HCN.

6. (5pts) Carbonate buffers are important in regulating the pH of blood at 7.40. What is the concentration ratio of CO₂ (usually written H₂CO₃) to HCO₃^- in blood at pH=7.40?

H₂CO₃(aq) HCO₃^-(aq) + H⁺(aq) K_a = 4.3 x 10^-7

7. (10 pts) HCN gas, a powerful respiratory inhibitor, is highly toxic. It is a very weak acid (K_a = 6.2 x 10^-10) when dissolved in water. If a 50.00-mL sample of 0.100 M HCN is titrated with 0.100 M NaOH, calculate the pH of the solution:

a. before the titration starts.

b. after 8.00 mL of 0.100 M NaOH has been added.

c. at the half-way point of the titration.

d. at the equivalence point of the titration.

Show all your steps for calculation questions in order to get full points.

1. (4 pts) The solubility product (K_sp) of the insoluble salt AgBr is 5.0 x 10^-13. Calculate the equilibrium constant for the following reaction: Ag⁺(aq) + Br^-(aq) AgBr(s).

2. (6 pts) A solution is prepared by adding 50.0 mL of 0.050 M HCl to 150.0 mL of 0.10 M HNO₃.

a. Calculate the pH of the solution.

b. Calculate the concentrations of all species in this solution.

3. (4 pts) Calculate the volume of concentrated (12 M) HCl needed to prepare 1-L of a pH = 1.50 solution?

4. (5 pts) Arrange the following 0.10 M solutions in order of most acidic to most basic.

KOH, KCl, KCN, NH₄Cl, HCl

5. (6 pts) Calculate the pH of an aqueous solution containing 1.0 x 10^-2 M HCl and 1.0 x 10^-2 M HCN.

6. (5pts) Carbonate buffers are important in regulating the pH of blood at 7.40. What is the concentration ratio of CO₂ (usually written H₂CO₃) to HCO₃^- in blood at pH=7.40?

H₂CO₃(aq) HCO₃^-(aq) + H⁺(aq) K_a = 4.3 x 10^-7

a. before the titration starts.

b. after 8.00 mL of 0.100 M NaOH has been added.

c. at the half-way point of the titration.

d. at the equivalence point of the titration.

e. after 52.00 mL of 0.100 M NaOH has been added.

8. (5 pts) Calculate the K_sp value. For bismuth sulfide (Bi₂S₃), which has a solubility of 1.0 x 10^-15 mol/L at 25â°C.

9. (5 pts) Arrange the following group of salts: in order of most soluble to most insoluble.

AgI (K_sp= 1.5 x 10^-16), CuI (K_sp= 5.0 x 10^-16) and CaSO₄ (K_sp= 6.1x 10^-5)

e. after 52.00 mL of 0.100 M NaOH has been added.

8. (5 pts) Calculate the K_sp value. For bismuth sulfide (Bi₂S₃), which has a solubility of 1.0 x 10^-15 mol/L at 25â°C.

9. (5 pts) Arrange the following group of salts: in order of most soluble to most insoluble.

AgI (K_sp= 1.5 x 10^-16), CuI (K_sp= 5.0 x 10^-16) and CaSO₄ (K_sp= 6.1x 10^-5)

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University College - Chemistry Chem 112A Lecture Notes - Lecture 9: Buffer Solution, Ph

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