NAME: _______________________________

Chemistry 30

Mr. de Bruin

Unit 3 Assignment.

All work must be fully shown in all questions to get processmarks.

As this is a summative assessment item, questions of clarificationmay be asked, but the teacher will not work through thequestions with students.

Chemistry 30 Unit 3 Assignment P a g e | 1T. de Bruin

Question 1

Use the following information to answer the question.

A chemist performed three trials of an experiment involving a closed system at equilibrium at700 K. The system can be represented by the equation

H2 (g) + I2 (g) � 2 HI (g) ΔH = –6.70 kJ

The chemist recorded the equilibrium concentrations of each gas in the table shown below.

Equilibrium Concentration

Trial H2 (g) mol/L I2 (g) mol/L HI (g) mol/L

1 0.165 0.0978 0.945

2 0.103 0.179 1.013

3 Unknown Unknown 1.55

a. Write the equilibrium constant expression for the formation of HI (g)

b. Use the information from trials 1 and 2 to calculate an average value for the equilibriumconstant, Kc, for the formation of HI (g).

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c. In trial 3, the equilibrium was established by allowing a sample of HI (g) to decompose.Calculate the equilibrium concentration of H2 (g) and I2 (g) for trial 3.

d. Identify a stress that would increase the value of the equilibrium constant and explain howthe stress is responsible for the increase.

Chemistry 30 Unit 3 Assignment P a g e | 3T. de Bruin

Question 2

Use the following information to answer the question.

A technician prepares 500 mL of a 0.100 mol/L solution of acetic (ethanoic) acid by diluting aconcentrated solution of acetic acid with water. The equilibrium equation that represents thereaction that occurs when acetic acid and water react is shown below.

CH3COOH (aq) + H2O (l) � CH3COO1– (aq) + H3O+ (aq)

a. For the prepared acetic acid solution, calculate the concentration of H3O+ (aq) and the pH.

Chemistry 30 Unit 3 Assignment P a g e | 4T. de Bruin

Use the following additional information to answer the next part of the question.

A technician uses the 500 mL of 0.100 mol/L acetic acid to prepare an acetic acid–sodiumacetate buffer solution. She does this by adding 0.100 mol/L sodium acetate to the 0.100 mol/Lacetic acid until the required pH is reached.

b. Identify the shift in equilibrium that will occur when sodium acetate is added to theequilibrium reaction. Predict whether or not there is an effect on the equilibrium constantand if so, what the effect on the equilibrium constant is.

c. If small quantities of aqueous sodium hydroxide are added to the buffer solution, predictwhether the pH of the acetic acid–sodium acetate buffer solution will change or stay thesame. Explain your answer.

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Question 3

Use the following information to answer the first question.

A student titrated 50.0 mL of 0.100 mol/L NH3 (aq) with 0.100 mol/L HCl (aq). The studentused a pH meter to measure the initial pH and the pH after each addition of HCl (aq). Thestudent’s data are recorded in the table.

Volume of HCl(aq)

(mL)pH

Volume of HCl(aq)

(mL)pH

0.0 11.12 50.0 5.27

10.0 9.83 50.1 4.00

20.0 9.41 51.0 3.00

30.0 9.06 60.0 2.04

40.0 8.63 70.0 1.78

49.0 7.55 80.0 1.64

49.9 6.54

a. Plot the data given above on the grid below and draw a titration curve. Identify the bufferregion.

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b. Determine the pH at the equivalence point. Provide an explanation for the pH value at theequivalence point. (Your explanation may be qualitative or quantitative).

c. Select a suitable indicator for this titration and explain why phenol red would not be agood choice.

Chemistry 30 Unit 3 Assignment P a g e | 7T. de Bruin

Question 4

Use the following information to answer the question.

HOCl (aq) a weak acid, is the active ingredient used in the disinfecting of swimming pools. Itcan be formed by adding Ca(OCl)2 (s) tablets to pool water.

The pH of a swimming pool should be kept between 7.2 and 7.8 so that the equilibrium [HOCl(aq)] is optimal. Phenol red is used by lifeguards to test pH. Based on the test results withphenol red, a lifeguard may

· Adjust the pH by adding Na2CO3 (s)· Adjust the pH by adding HCl (aq)· Not do anything

a. Write the net ionic equation that illustrates the formation of HOCl (aq) when Ca(OCl)2 (s)tablets are added to a swimming pool.

b. Identify two characteristics of this system, or of any system, at equilibrium.

c. Based on the indicator colour, the lifeguard may choose any one of the three differentcourses of action. Relate each indicator colour to a course of action.

Chemistry 30 Unit 3 Assignment P a g e | 8T. de Bruin

Question 5

An unidentified acid with a concentration of 1.00 mol/L has been given to you to identify. Theacid appears in your data booklet on the Relative Strengths of Acids and Bases Table. Thefollowing test results were recorded.

1. Methyl violet is yellow when added to the acid.2. The acid did not form a precipitate when a solution containing Ag+ (aq) was added to it.3. The solution turned blue and a gas was formed when a strip of copper was added to the

acid.

Based on these test results, identify the acid and justify your choice. Your answer shouldinclude equations and/or calculations where appropriate.

Chemistry 30 Unit 3 Assignment P a g e | 9T. de Bruin

Question 6

A student needed to determine the concentration of a hydrochloric acid solution. To titratesamples of the hydrochloric acid solution she prepared 100.0 mL of a standard solution ofsodium oxalate using 1.85 g of the dry solid. Using the second endpoint, 10.0 mL samples weretitrated with the hydrochloric acid solution.

EvidenceTable 1 Titration of 10.0 mL Samples of Na2OOCCOO(aq) with HCl(aq)

Trial 1 Trial 2 Trial 3 Trial 4Final burettereading (mL) 16.1 31.5 46.9 16.9

Initial Burettereading (mL) 0.3 16.1 31.5 1.5

Volume HCl(aq) used

(mL)

a. Calculate the concentration of the hydrochloric acid solution

b. Determine the pH of the hydrochloric acid solution.