17.1-17.3 Quiz

What is a buffer?

consists of a mixture of a weak acid and its conjugate base

How to determine salt & weak acid that make buffers?

common ion + WB and WA; are not strong acids/bases

Equilibrium equation for a weak acid

WA + H₂O ←→ H₃O⁺ + A^-

How does the addition of an acid to an equilibrium will affect the concentrations of ions?

H⁺ is added and X^- is consumed to produce HX; pH does not significantly change; shifts to produce more WA

How does the addition of a base to an equilibrium will affect the concentrations of ions?

OH^- reacts with HX to produce X^- and water; no significant pH change; more conjugate base (X^-) is made

Henderson-Hasselbalch equation

use to relate pK_a to pH and pH to the initial concentration of the WA and CB

When are pH and pK_a equal?

at ½ of the equivalence point

What is the relationship between pH and pK_a?

pH is within ± 1 unit of pK_a in a (ideal) buffer

How can you determine the combination of WA and CB that has the greatest buffering capacity?

the greater amounts of conjugate acid-base pair (molar concentration); most effective within 1 pH unit of pK_a

Buffer Capacity

the amount of acid or base neutralized by the buffer before there is a significant change in pH

SA/SB titrations curve

initial pH = 1; sharp and long Inflection points; equivalence point is 7

WA/SB titrations curve

initial pH between 2 and 3; excess of acid before equivalence point; smaller Inflection point; equivalence point around 8

Stronger the acid with _____ the inflection point

longer

How do you determine the initial concentration of the given solution?

WA- use K_a to determine initial pH since solution only contains weak acid; pH of the dissociation of the acid/base

Given a list of indicators, which would be the best one for the titration?

dramatic color change in the desired range; phenolphthalein

How do you determine the hydronium ion concentration or pH of the solution after x mL of acid or base has been added to the solution of known volume and molarity?

do neutralization; find moles of each; BCA charts to determine what is in excess; find total volume; find molarity of excess substance; find pH or pOH then pH

What is the pH at the equivalence point in the titration?

SB/SA- pH = 7.00; WA/WB- pH > 7.00 (around 8.00)

titration curve

plot of pH vs. volume of acid/base added

equivalence point

point at which the acid and base are present in stoichiometric quantities

End point

observed point

titration error

different between equivalence point and end point

Polyprotic Acid Titrations

each ionizable proton dissociates in steps; n equivalence point corresponding to each ionizable proton