chem ch 14

Which of the following statements concerning phenolphthalein are true? 1. Phenolphthalein changes from colorless in acidic solutions to pink in basic solutions. 2. Phenolphthalein is obtained from red cabbage juice. 3. Phenolphthalein changes color at about pH 9.

1 AND 3

Bromthymol blue Ka = ______________

1 X 10-7

Strong acid-strong base: pH = ______________

7; neutral salt in water

Strong acid-weak base: pH = ____________

< 7; weak acid in water

Weak acid-strong base: pH = ______________

>7; weak base in water

All of the following statements are false for the titration of a strong base by a strong acid EXCEPT a. the equivalence point occurs at pH 7. b. there will be at least two equivalence points, one for the base and one for the acid. c. the equivalence point and the end point are identical, regardless of what indicator is used. d. equal masses of acid and base are required to neutralize the solution. e. phenolphthalein, if used as an indicator, will turn from colorless to pink.

A

we can prepare a buffer by:

A weak acid, HB The conjugate base, B-, as a sodium salt, NaB Recall that Na+ is a spectator ion so it does not affect pH

Which of the following statements concerning acid-base indicators are true? 1. Acid-base indicators are derived from weak acids. 2. The acid and base forms of the indicator are different colors. 3. Only small quantities of indicator are added to a solution.

ALL

If Hln/ln- is around 1, the indicator will be ___________________

An intermediate color

What is a buffer?

Any solution containing appreciable amounts of both a weak acid and its conjugate base Is highly resistant to changes in pH brought about by the addition of a strong acid or base Has a pH close to the pKa of the weak acid A SUBSTANCE THAT PREVENTS OR RESISTS A CHANGE IN PH

with Bromthymol blue, as pH increases from 0-14:

At pH 6, the indicator is yellow Between pH 6 and 7, the color changes to green At pH 7, we have a green color Between pH 7 and 8, the green changes to blue At pH 8 (and above) the indicator is blue

Which of the following solutions has the greatest buffer capacity? a. A 1.0 L aqueous solution of 1.00 M HCl b. A 1.0 L mixture of 1.00 M H3PO4 and 1.00 M H2PO4− c. A 1.0 L mixture of 1.00 M H3PO4 and 0.500 M NaOH d. A 1.0 L mixture of 1.00 M H2PO4− and 1.00 M HCl e. A 1.0 L aqueous solution of 0.75 M H2SO4

B. A 1.0 L mixture of 1.00 M H3PO4 and 1.00 M H2PO4−

Why will the equivalence point of a weak acid/strong base titrationl be basic?

Because the product of the titration is a weak base

Which indicator is most appropriate for the titration of acetic acid with NaOH? The Ka for CH3CO2H is 1.8 × 10−5. a. methyl red (pH 5) b. bromthymol blue (pH 7) c. phenolphthalein (pH 9) d. Both methyl red and bromthymol blue are suitable. e. All three indicators are suitable.

C

Which of the following solutions has the least buffer capacity? a. A 1.0 L mixture of 0.100 M H3PO4 and 0.100 M H2PO4− b. A 1.0 L mixture of 0.200 M H3PO4 and 0.100 M NaOH c. A 1.0 L mixture of 0.200 M H2PO4− and 0.100 M HCl d. A 1.0 L mixture of 0.200 M H2PO4− and 0.100 M HNO3 e. All of the above have the same buffer capacity.

E

Which one of the following conditions is always true for a titration of a weak acid with a strong base? a. A colored indicator with a pKa less than 7 should be used. b. If a colored indicator is used, it must change color rapidly in the weak acid's buffer region. c. Equal volumes of weak acid and strong base are required to reach the equivalence point. d. The equivalence point occurs at a pH equal to 7. e. The equivalence point occurs at a pH greater than 7.

E

Why is the buffer capacity to react with acid or base limited?

Eventually, all the HB reacts with OH- Eventually, all the B- reacts with H+

What is the fundamental acid equation?

H+ (aq) + B- (aq) ⇌ HB (aq)

Natural rainfall is slightly acidic at a pH of about 5.5. Why is the pH of natural rain acidic?

H2CO3 forms when CO2 dissolves in rainwater

Which two acids are responsible for acid rain?

HNO3 and H2SO4

what is the formula for an indicator?

Hln

what is an indicator?

Indicators are weak organic acids with a special property They are one color in acid and ... Another color in base

The pH of a buffer depends on two factors:

Ka for the acid: if HB and B- are present in nearly equal amounts, pH = pKa The ratio of the concentration or amounts of HB and B- Adding more base than a 1:1 will make the buffer more basic

What is the fundamental base equation?

OH- (aq) + HB (aq) ⇌ B- (aq) + H2O

what would be a good choice for the weak acid/strong base titration?

Phenolphthalein, with endpoint pH = 9, is a good choice for this titration

What is the effect on pH when a buffer is diluted by a factor of 10?

The buffer pH does not change appreciably.

What is the buffer range?

The buffer range is the pH range over which the buffer is effective

What is the effect of adding NaOH to an aqueous solution of ammonia?

The pH of the solution will increase, the concentration of NH4 will decrease and the concentration of NH3 will increase

what are the features of a strong acid/strong base titration

The pH starts out very low There is a gradual rise in pH as base is added Near the equivalence point, the pH rises sharply Most of the acid has been neutralized After the equivalence point, the pH rises slowly as more base is added to the titration mixture The K for this reaction is 1/Kw or 1 X 1014

The presence of a conjugate base in a buffer serves what purpose?

The presence of B- gives added H+ a reactant: B- (aq) + H+ (aq) ⇌ HB (aq) + H2O

The presence of a weak acid in a buffer serves what purpose?

The presence of HB gives added OH- a reactant: HB (aq) + OH- (aq) ⇌ B- (aq) + H2O

Two factors control the color of the indicator and the pH at which it will change color:

The ratio of [HIn]/[In-] The Ka of the indicator

How does a buffer resist large pH changes?

The strong base is converted to a weak one by the buffer The strong acid is converted to a weak one by the buffer

Bromthymol blue is ____________ in acid ___________ in base

Yellow; Blue

The color of the indicator is controlled by _______________, which determines ________________

[H+]; [HIn]/[In]

Partial neutralization of _____________________________ will produce a buffer

a weak acid by a strong base

Partial neutralization of ___________________ will also produce a buffer

a weak base by a strong acid

The pH at the equivalence point of a strong acid-weak base titration is _______________

acidic

All of the following statements concerning acid-base buffers are true EXCEPT a. buffers are resistant to pH changes upon addition of small quantities of strong acids or bases. b. buffers are used as colored indicators in acid-base titrations. c. the pH of a buffer is close to the pKa of the weak acid from which it is made. d. buffers contain appreciable quantities of a weak acid and its conjugate base. e. buffers are resistant to changes in pH when diluted with water.

b

There must be _____________________ in order to produce a buffer

both species present

In choosing an indicator for the weak acid/strong base titration, we need one that will _________________________

change color at basic pH

Which of the following pairs will form a buffer when mixed together in an aqueous solution? a. KCl and KH2PO4 b. HCl and KOH c. Ca(OH)2 and NaOH d. HF and NaF e. None of the above will form a buffer.

d

Which one of the following reactions describes the attack of acid rain on limestone? a. 2H+(aq) + Ca(s) → Ca2+(aq) + H2(g) b. 2H+(aq) + Ca(OH)2(s) → Ca2+(aq) + 2H2O(l) c. 2H+(aq) + CaO(s) → Ca2+(aq) + H2O(l) d. 2H+(aq) + CaCO3(s) → Ca2+(aq) + CO2(g) + H2O(l) e. 3H+(aq) + Al(OH)3(s) → Al3+(aq) + 3H2O(l)

d. 2H+(aq) + CaCO3(s) → Ca2+(aq) + CO2(g) + H2O(l)

If the acid to base ratio in a buffer increases by a factor of 10, the pH of the buffer

decreases by 1

When mixed together, all of the following pairs can form buffers EXCEPT a. H3PO4 and NaH2PO4. b. NaH2PO4 and Na2HPO4. c. CH3CO2H and NaOH. d. HCl and NaCH3CO2. e. NaI and NaOH.

e

All of the following solutions are buffers EXCEPT a. 100 mL of 0.500 M H2PO4− added to 100 mL of 0.200 M HPO42−. b. 100 mL of 0.200 M HF added to 200 mL of 0.200 M NaF. c. 200 mL of 0.200 M HCl added to 200 mL of 0.400 M CH3CO2−. d. 200 mL of 0.500 M NaOH added to 200 mL of 1.000 M HF. e. 200 mL of 0.400 M HCl added to 100 mL of 0.200 M CH3CO2−.

e. 200 mL of 0.400 M HCl added to 100 mL of 0.200 M CH3CO2−.

a buffer is made up of _______________________ and _____________________________.

equal amounts of the weak acid and a salt containing its conjugate base

In a titration of HCl(aq) with NaOH(aq), the equivalence point is the point at which

equal moles of HCl and NaOH have reacted.

When equal amounts of a weak acid and its conjugate base are present, the pH of the buffer is:

equal to pKa

What is a buffer capacity

how much the buffer can absorb and still resist the change in pH

What is the relationship between the pKa for an acid-base indicator and the pH at which the indicator changes color?

pH = pKa

What is the Henderson-Hasselbalch equation?

pH=pka + log [B-]/[HB]

the Buffer range is related to the:

ratio of HB/B-; The further the ratio is from 1:1, the less effective the buffer is and the shorter the buffer range

The buffer capacity is

the amount of H+ or OH− that can be absorbed by a buffer without changing the pH appreciably.

If Hln/ln- is greater to or equal to 10, the indicator will be _________________

the color of the acid

If Hln/ln- is less than or equal to .10, the indicator will be _______________

the color of the base

In order to make a buffer from a strong base and a weak acid (or a strong acid and a weak base), the strong acid (or strong base) must be ______________________________________________

the limiting reactant. Putting it another way, you must have more moles of the weak acid/weak base component than the strong acid/strong base component

Equilibrium for HIn is ______________________

the same as for any other weak acid HIn (aq) H+ (aq) + In- (aq) ka= [H+][ln-]/[Hln]

The pH at the equivalence point is controlled by __________________

the species present

In a titration experiment, if the initial solution pH is 4.0 and the equivalence point occurs at pH 9.0, then the reaction corresponds to

the titration of a weak acid by a strong base.

The buffer + acid and buffer + base reactions both demonstrate ___________________, and __________________

very large equilibrium constants; go nearly to completion