Buffer Solutions, Function

18.2 Know the following about a chemical buffer: Function Components Design Buffer action

Buffer Solutions, Buffer Solutions, FunctionFunctionHCl is added to pure HCl is added to pure

water.water.

HCl is added to a HCl is added to a solution of a weak solution of a weak acid Hacid H22POPO44

-- and its and its conjugate base conjugate base HPOHPO44

2-2-..

Components of a buffer A Weak acid and its Conjugate base

HA + H2O H3O+ + A-

HA H+ + A-

HOAcHOAc + H + H22O HO H33OO++ + + OAcOAc--

Acetic Acid/Acetate Buffer

HOAcHOAc H H++ + + OAcOAc--

The Metal Cation is not shownOAc-: CH3COO-

Buffer Design Rule of thumb: [HA] = [A-] [HA] & [A-] are least 0.10M each pH = pKa, use [HA], [A-] to tweak Use pH = pKa + log[A-] [HA]

You want to buffer a solution at pH = 4.30 or You want to buffer a solution at pH = 4.30 or

[H[H33OO++] = 5.0 x 10] = 5.0 x 10-5-5 M M

POSSIBLE ACIDSPOSSIBLE ACIDS KKaa

HSOHSO44- - / SO/ SO44

2-2- 1.2 x 10 1.2 x 10-2-2

HOAc / OAcHOAc / OAc-- 1.8 x 101.8 x 10-5-5

HCN / CNHCN / CN-- 4.0 x 104.0 x 10-10-10

Best choice is…? What are the pKaBest choice is…? What are the pKass? ?

What are the molarities of HA, AWhat are the molarities of HA, A--??

Preparing a BufferPreparing a Buffer

HA/A- Ka pKa

HSO4- / SO4

2- 1.2 x 10-2 1.92 HOAc / OAc- 1.8 x 10-5 4.74 HCN / CN- 4.0 x 10-10 9.40

Use pH = pKa + log[A-] [HA]

pH = 4.74 + log(0.1/0.1) = 4.74

Buffer action

HOAcHOAc H H++ + + OAcOAc--

Reacts with added base (OH-)

Reacts with added acid (H+)

Acid-Base TitrationsAcid-Base Titrations

Adding NaOH from the buret to acetic acid in the flask, a weak acid. Adding NaOH from the buret to acetic acid in the flask, a weak acid. In the beginning the pH increases very slowly.In the beginning the pH increases very slowly.

Additional NaOH is added. pH rises as equivalence point is approached.Additional NaOH is added. pH rises as equivalence point is approached.

Additional NaOH is added. pH increases and then levels off as Additional NaOH is added. pH increases and then levels off as NaOH is added beyond the equivalence point.NaOH is added beyond the equivalence point.

18.3 Know how to calculate the pH at different points during a titration

Know how to use the following terms: Equivalence point

Buffer region

Know how to draw titration curves for: Strong Acid/Strong Base Weak Acid/Strong Base Strong Acid/Weak Base

Strong Acid Strong Base Reaction

TitrationsTitrationsTitrationsTitrations

Know how to select the appropriate indicator for a titration

Indicators for Acid-Base TitrationsIndicators for Acid-Base TitrationsIndicators for Acid-Base TitrationsIndicators for Acid-Base Titrations

18.4 Know that Ksp is the equilibrium constant for precipitates

Know how to determine Ksp Know how to calculate the solubility of precipitates using Ksp

Know how to calculate the solubility of precipitates considering the common ion effect

Some Values of KSome Values of Kspsp

Table 18.2 and Appendix JTable 18.2 and Appendix J

PRECIPITATION REACTIONSPRECIPITATION REACTIONSSolubility of PrecipitatesSolubility of Precipitates

PRECIPITATION REACTIONSPRECIPITATION REACTIONSSolubility of PrecipitatesSolubility of Precipitates

Lead(II) iodide

Lead(II) ChlorideLead(II) ChloridePbClPbCl22(s) Pb(s) Pb2+2+(aq) + 2 Cl(aq) + 2 Cl--(aq) (aq)

KKspsp = 1.9 x 10 = 1.9 x 10-5-5 = [Pb = [Pb2+2+][Cl][Cl––]]22

Barium Barium

SulfateSulfate

KKspsp = 1.1 x 10 = 1.1 x 10-10-10

(b) BaSO4 is opaque to x-rays. Drinking a BaSO4 cocktail enables a physician to exam the intestines.

(a) BaSO4 is a common mineral, appearing a white powder or colorless crystals.

Common Ion EffectCommon Ion EffectPbClPbCl22(s) Pb(s) Pb2+2+(aq) + 2 Cl(aq) + 2 Cl--(aq) (aq)

KKspsp = 1.9 x 10 = 1.9 x 10-5-5

Did I cover these? Use the common ion effect to predict

relative solubility. Use LeChatelier’s principle to predict the

change in solubility of an ionic solid. Given an ion, formulas containing the ion,

and molarities of the ion determine the Ksp for each compound.