Lab 5

Question 1

what will you be doing In this lab

Answer 1

determining the identity of a weak diprotic acid based on the molecular mass
obtained from a titration with standardized NaOH (strong base) and also calculate the acid dissociation
constants, Ka1 and Ka2.

Question 2

what is a diprotic acid

Answer 2

A diprotic acid is an acid that yields two H+ ions per acid molecule.

[h2so4, h2co3]

Question 3

how can we determine Ka

Answer 3

by observing the changes in pH that occur when a diprotic
acid is titrated with a base, such as NaOH,

Question 4

first-titration point

Answer 4

one half of the protons have been titrated with OH

[H2X]=[HX-]

Question 5

second-titration point

Answer 5

one half of the protons have been titrated with OH

[HX-]=[X^2-]

Question 6

pH at the first-titration point is

Answer 6

equal to pKa1

Question 7

first half-
titration point volume can be founds by …

Answer 7

by dividing the first equivalence point volume by two.

Question 8

Similarly,
the pH at the second half-titration point is equal to ….

Answer 8

• pH at the second half-titration point is equal to pKa2
• second half titration volume is midway between the first and second equivalence point volume

Question 9

Solutions of NaOH are virtually impossible to prepare to a precise molar concentration because the
substance

Answer 9

is hygroscopic [absorbs moisture]

• its never 100% NaOH

Question 10

The process of determining the exact concentration of
one solution by titration with a solution of known, precise concentration

Answer 10

standardization

Question 11

stages of changes to pH

Answer 11

stage 1:
- pH of acidic is initially low

stage 2:
- change in pH is gradual until its close to equivalence point when equal amounts of acid and base are mixed

stage 3:
- near equivalence point, pH will increase quickly and then becomes more gradual until it levels off with excess base

Question 12

How can the change in pH be monitored in this experiment

Answer 12

This change in pH will be monitored with a pH meter.
The region of most rapid pH change will then be used to determine the equivalence point.

Question 13

pKa1 vs pKa2

Answer 13

pKa1: pH that corresponds to the volume of NaOH required to reach the 1st half equivalence point

pKa2: pH that corresponds to the midpoint between the 1st and 2nd equivalence point