CHEM 0970 Lecture Notes - Lecture 5: Buffer Solution, Rice Chart, Equivalence Point

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27 Apr 2016

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Chapter 17: acid-base equilibria buffers & titrations. Defiitio: a aqueous solutio cotaiig a weak acid ad its cojugate base or a weak base ad its cojugate acid. Ezymes ad proteis oly work correctly at a very arrow ph rage. Buffers are also used i fermetatio processes, chemical aalysis, ad i dye settig. Very commo buffer: pbs (phosphate-buffered salie) ph = 7. 4. They miimize ph chages by reactig with either acids or bases ex: a buffer of hc7h5o2 & nac7h5o2 if you add acid, the basic salt reacts with it: The acid is eutralized (reacted); a weak acid is produced, but it ioizes much less tha a strog acid would if you add base, the acid reacts with it: The base is eutralized (reacted); the aio made will deprotoate water, makig.

Related Questions

For the dissociation reaction of a weak acid in water,

HA(aq)+H2O(l)âH3O+(aq)+Aâ(aq)

the equilibrium constant is the acid-dissociation constant, Ka, and takes the form

Ka=[H3O+][Aâ][HA]

Weak bases accept a proton from water to give the conjugate acid and OHâ ions:

B(aq)+H2O(l)âBH+(aq)+OHâ(aq)

The equilibrium constant Kb is called the base-dissociation constant and can be found by the formula

Kb=[BH+][OHâ][B]

When solving equilibrium-based expression, it is often helpful to keep track of changing concentrations is through what is often called an I.C.E table, where I. stands for Initial Concentration, C. stands for Change, and E. stands for Equilibrium Concentration. To create such a table, write the reaction across the top creating the columns, and the rows I.C.E on the left-hand side.

Initial (M)Change (M)Equilibrium (M)A+ BâAB

Part A

Aspirin (acetylsalicylic acid, C9H8O4) is a weak monoprotic acid. To determine its acid-dissociation constant, a student dissolved 2.00 g of aspirin in 0.600 L of water and measured the pH. What was the Ka value calculated by the student if the pH of the solution was 2.60?

Express your answer numerically using two significant figures.

Hints

Ka =

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You used the initial concentration of aspirin in the Ka expression. Instead, you need to use the equilibrium concentration. Consider that the amount of aspirin that reacted is equal to the amount of H₃O⁺ produced. How much is left over?

Part B

A 0.100 M solution of ethylamine (C2H5NH2) has a pH of 11.87. Calculate the Kb for ethylamine.

Express your answer numerically using two significant figures.

Hints

Kb =

Fill out the whole table, you do not have to show your work on all of them, just show your work on 4 different kinds, example:

Problem #1: an acid that I measured the pH, we turn that into hydrogen ion concentration and fill out the table.
Problem #2: an acid where I did not measure the pH, in which case you start by looking up kA, then fill out the table
Problem #3: a base where I measured the pH turn that into hydroxide concentration and fill out the table,
Problem #4: a base where I did not measure the pH, then you have to find kB and fill out the table.

You only have to show work for each one of these and the rest can be done without any work.

In that table if you were to look at "Exp pH" you could see that some of them are crossed over and another pH is written, those ones are called "measured" for the measured columns you have to fill out "Theo Ka or Kb" and the "%error" as well. For the non measured rows, you do not have to fill out "Theo Ka or Kb" and the "%error."

NOTES:
1) For the Conjugate Acid and Conjugate Base, do not put H+, OH-. If itâs a weak acid, put that weak acid. For Conjugate base, then put the weak base. If the compound was a weak base, then put that thing in there, then put the conjugate acid in the other column. And do not include any spectator ions.

2) Do not bother to use the ice table to determine X, values for Ka or Kb can be looked up online.

3) In the case of nitrate ion, we cannot Kb, we can only look up Ka for its conjugate acid.

4) 7 of them have their pH measured. And the other 7 do not and that should be looked up.

CHEM 0970 Lecture Notes - Lecture 5: Buffer Solution, Rice Chart, Equivalence Point

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