Experiment 4: pH Chemistry: Acid-Base Titrations and Buffers

Question 1

pH =

Answer 1

-log[H+]

Question 2

How does the pH meter indicate pH?

Answer 2

By measuring electromotive force (emf) developed by a cell, using the solution as an electrolyte.

Question 3

What is the acid strength of a Bronsted-Lowry acid? If an acid is strong, is its conjugate base strong?

Answer 3

The ability of the acid to relinquish protons. A strong acid gives up protons more readily than a weak acid. If an acid is strong, its conjugate base is weak and does not readily accept protons.

Question 4

What is a buffer?

Answer 4

A solution that resists changes in pH by shifting equilibrium between a conjugate acid-base pair.

Question 5

How can the pH of a buffer system be calculated?

Answer 5

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

Question 6

What is the buffer ratio

Answer 6

[A-]/[HA]
- the pH of a solution is depends directly on the log of the buffer ratio (if pH increases the buffer ratio increases)

Question 7

What is buffer capcity?

Answer 7

The number of moles of strong acid/base that is needed to change the pH of 1L of buffer by 1 pH unit.

Question 8

How is the buffer capacity affected by the buffer ratio?

Answer 8

When the buffer ratio is 1, i.e. pH = pKa, the buffer is most resistant to change, and has the highest buffer capacity.

Question 9

how do acid-base indicators work and how to select an appropriate indicator

Answer 9

• Acid-base indicators are substances that change colour depending on the pH of the solution they are in.
• They are often used in titrations and other chemical experiments to visually determine the endpoint of an acid-base reaction
• When the pH of the solution changes, the equilibrium shifts, causing a change in the relative concentrations of the acid and conjugate base forms of the indicator. This, in turn, results in a change in the color of the indicator.
• Each indicator has its own pH range and color change characteristics, making them suitable for different types of acid-base titrations and experiments

Question 10

what is the end point

Answer 10

the point in the titration at which an indicator changes color

Question 11

what is a titration curve

Answer 11

graphical representation of the pH (or another relevant property) of a solution being titrated plotted against the volume of titrant added

Question 12

what is the equivalence point

Answer 12

the point where the number of moles of titrant added is equal to the number of moles of analyte present in the solution being titrated.

Question 13

what is the half neutralization point

Answer 13

a specific point in a titration curve where exactly half of the analyte has been neutralized by the titrant

Question 14

what is the indicator transition range

Answer 14

• the pH range over which an acid-base indicator undergoes a significant and observable color change
• determined by the pKa value of the indicator and indicates the pH range in which the indicator transitions between its acid (HIn) and conjugate base (In⁻) forms

Question 15

what makes one buffer better than the other

Answer 15

• the buffer capacity is highest where pH = pKa
• weak acids or bases that partially dissociate make better buffers
• resist a change in pH after adding an acid or a base

Question 16

how does a buffer maintain a steady ph

Answer 16

able to neutralize small amounts of added acid or base, thus maintaining the pH of the solution relatively stable