Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 1

Chemistry 3202 Unit 2 Section 3 Homework Portfolio - Key 2006

1. Alizarin yellow R indicator (abbreviated HAy) has an approximate pH range of

10.1 to 12.0. The weak acid form is yellow and the conjugate base form is red. (2) (a) Write a complete ionization equation for Alizarin yellow R indicator in water.

(aq) 2 (l) 3 (aq) (aq)HAy H O H O Ay

Yellow Re d

+ −+ +

(2) (b) Which form of the Alizarin yellow R indicator has a higher concentration in a 1.00

x 10-5 M NaOH solution? Explain. Since the [NaOH] = [OH-] = 1.00 x 10-5 M

5

(aq)pOH log[OH ] log(1.00 10 M) 5.00

pH 14.00 pOH 14.00 5.00 9.00

− −= − = − × =

= − = − =

At this pH value the Alizarin yellow indicator displays the color of the weak acid, yellow.

(2) (c) Two drops of Alizarin yellow R have been added to a 0.100 M NaOH solution.

Name a compound that can be added to the mixture to cause the colour of the indicator to change from red to yellow. Justify your choice with reference to Le Châtelier’s principle.

When added to the very basic NaOH solution the indicator equilibrium will shift to the right

as the hydroxide ions present will neutralize the hydronium ions present and cause more of the weak acid to ionize. This will result in the formation of the red conjugate base species and the solution will display the red color. The addition of a strong acid will cause a change in the color from red to yellow. The addition of hydronium ions will cause the reverse reaction to be favored, and thus shift the indicator equilibrium to the left.

(2) (d) Alizarin yellow R is added to a solution whose pH is not known. The colour

observed is yellow. When another sample of the solution is tested with chlorophenol red, it becomes red, and when tested with metacresol purple indicator a sample turns purple. What is the approximate pH of the solution? Provide a full rationale for your answer.

Observation Conclusion Alizarin Yellow is yellow pH < 10.1 Chlorophenol red becomes red pH > 6.8 Metacresol purple turns purple pH > 9.0 OVERALL CONCLUSION: Solution pH lies between 10.1 > pH > 9.0

Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 2 (2) 2. (a) What is a titration? A titration is the progressive addition of one reactant to another. The process is

normally done to determine the unknown concentration of a sample. (2) (b) Sketch and label a typical titration apparatus.

A typical titration involves the release of aqueous solution from a buret into a sample flask. The sample flask contains the sample solution and an indicator which will change color to signify the endpoint of the titration.

(2) (c) Why is it common to overshoot the endpoint on the first trial? When doing a titration it is not known when the sample will be neutralized. When

doing the first trial of a titration it is common to add too much titrant, to overshoot the endpoint. This first trial, however, does give the researcher an indication of the approximate amount of titrant needed to neutralize the sample, and thus allows the researcher to more precisely neutralize the sample on subsequent trials.

(2) (d) Distinguish between indicator endpoint and equivalence point.

The endpoint of a titration is the point at which the indicator changes color. The equivalence point is the point at which stoichiometric equivalent amounts of titrant and sample have reacted. It is important that the indicator be chosen so that its endpoint will occur at the equivalence point.

3. Biff and Molly prepare a 0.10 M NaOH solution by dissolving 4.00g NaOH in 1.00 L of water.

(2) (a) Is the 0.10 M NaOH a primary standard? Why? Why not? The solution of NaOH is NOT a primary standard. Solid sodium hydroxide is a

deliquescent solid, which means it absorbs water from the atmosphere. Since solid will contain an unknown amount of water, a solution of precise concentration cannot be prepared from the solid solute.

Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 3 (2) (b) Describe how to standardize a sodium hydroxide solution. Once prepared, a solution of sodium hydroxide can be standardized. This is

done by filling a buret with the sodium hydroxide solution and titrating the solution into a flask containing an acidic primary standard until the endpoint occurs. Several trials will provide for the calculation of the precise concentration of the sodium hydroxide solution; the ‘standardized’ sodium hydroxide solution.

4. Biff and Molly titrated several 25.00 mL samples of a certain potassium hydroxide

solution with standardized 0.1108 mol/L hydrochloric acid. These buret readings were obtained.

Trial 1 2 3 4 Initial Buret Reading (mL) 50.00 38.25 28.76 19.25 Final Buret Reading (mL) 38.25 28.76 19.25 9.78 Titrant Volume Used (mL) 11.75 9.49 9.51 9.47

(2) (a) Calculate the volume of titrant used in each trial, evaluate the titrant volumes,

and calculate an average volume. Discarding the first value, the average volume of hydrochloric acid, used is:

(9.49 9.51 9.47)mL9.49 mL

3+ +

=

(3) (b) Calculate the molar concentration of the potassium hydroxide solution.

(aq) (aq) (aq) (l)

3HCl

3 3

32

HCl KOH KCl HOH

v 0.00949 L v 0.02500 Lc 0.1108 M

n n[HCl] 0.1108M n 1.0514 10 mol HCl

v 0.00949L

1 mol KOH1.0514 10 mol HCl 1.0514 10 mol KOH

1 mol HCl

n 1.0514 10 mol[KOH] 4.206 10 M

v 0.02500L

−

− −

−−

+ → +

= ==

= = = ×

× × = ×

×= = = ×

(1) (c) What indicator would you suggest to Biff and Molly for this titration? Since this is a titration of a strong acid with a strong be I would expect the

endpoint to occur around pH = 7. Suitable indicators would include: Bromothymol blue (6.0-7.6), litmus (6.0-8.0), and phenol red (6.6-8.0).

Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 4 (3) 5. Calculate the mass of calcium carbonate required to neutralize 5.0 x 106 L of acidified

pond water that has a pH of 3.44. The equation for the neutralization reaction is: From the pH we can calculate the [H3O+]: pH 3.44 4

3 (aq)[H O ] 10 10 3.63 10 M+ − − −= = = ×

3

+ 2+3 (s) 3 (aq) (aq) 2 (l) 2(g)

6

4

+ 43 (aq) 6

33 3

3

CaCO 2 H O Ca 3 H O CO

? 5.0 10

3.63 10

[H O ] 3.63 10 18155.0 10

11815 907.5

2

907.5 90832 90.8100.09 /

+

−

−

++

+ → + +

= = ×

= ×

= × = =×

× =

= = = =

H O

m v L

c M

n nM n mol

v L

mol CaCOmol H O mol CaCO

mol H O

m mn mol m g kg

M g mol

(5) 6. Calculate the pH of a mixture formed by mixing a 2.00 L solution containing 1.22g of strontium hydroxide, Sr(OH)2, with a 500.0 mL solution of 0.128 mol/L nitric acid.

3

2(s) 3(aq) 3 2(aq) (l)

OH H O

3

3 3

Sr(OH) 2 HNO Sr(NO ) 2 HOH

v 2.00 L v 0.5000 Lm 1.22 g c 0.128 Mn 0.0100 mol n 0.0640 mol

strong base strong acidn 0.0200 mol n 0.0640 mol

EXCESS Re ac tant H O :

0.0640 mol H O 0.0200mol H O 0.0440mo

− +

+

+ +

+ → +

= == == =

= =

=

− = 3

33

3

l H O in excess!

0.0440mol H On[H O ] 0.0176 M

v 2.50 L

pH log[H O ] log(0.0176M) 1.75

+

++

+

= = =

= − = − =

Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 5 7. For each acid-base reaction: (3) (a) hydrofluoric acid titrated against sodium hydroxide.

(3) (b) nitric acid titrated with potassium hydroxide.

(3) (c) perchloric acid titrated with sodium hypochlorite

Chemistry 3202 Unit 2 Section 3 - HWP 2006 page 6 (2) 8. (a) Using examples, distinguish between polyprotic and polybasic species. Polyprotic acids are acids that can donate more than one proton. Examples

would include; H2SO4, H3PO4 Polybasic species are bases that can accept more than one proton.

Examples would include ions such as: the sulfide ion, S2- and the oxalate ion, OOCCOO2-

(3) (b) Write the reaction steps and the overall reaction for the reaction between aqueous potassium sulfite and excess hydrobromic acid.

Potassium sulfate dissociates to release the sulfate ion. Hydrobromic acid is a strong acid that ionizes to form the hydronium ion.

23(aq) 3 (aq) 3(aq) 2 (l)

3(aq) 3 (aq) 2 3(aq) 2 (l)

23(aq) 3 (aq) 2 3(aq) 2 (l)

Step 1: SO H O HSO H O

Step 2 : HSO H O H SO H O

Overall

Re action : SO 2 H O H SO 2 H O

− + −

− +

− +

+ → +

+ → +

− − − − − − − − − − − − − − − − − − − − − − − − − − − − −

+ → +

(2) (c) Sketch a titration curve for the reaction showing the approximate equivalence point(s).