Science Olympiad
Coach’s Clinic
CHEMISTRY LAB
Pt. 1
ACID-BASE STATIONS
• Students will be expected to answer
questions and/or interpret data related to
acids and bases; and their properties and
chemistry.
Sample Questions
1. If the following titration curve resulted from
titrating acid X with a strong base, what might
be the identity of acid X?
a) HCl b) H2CO3 c) HC2H3O2 d H2SO4
• 2. If a solution of ammonia is titrated with
nitric acid, the pH at the equivalence point
should be
• a) less than 7
• b) equal to 7
• c) greater than 7
• d) more information needed
• 3. If 20.00 mL of acid HX (a weak
monoprotic acid) required 32.50 mL of
standard 0.150M NaOH to reach the
endpoint, what is the molarity of the
original acid solution?
solution
• HX + Na+ + OH-  Na+ + X- + H2O
• .03250L x .1500M OH- = .004875 mol OH• .004875mol OH- x 1mol HX / 1mol OH- =
.004875mol HX
• .004875mol HX/.02000L = .2438M HX
• 4. What is the pH of the original HX
solution if the Ka is 1.81x 10-5 ?
solution
•
•
•
•
•
•
HX + H2O  X- + H3O+
Init.
Δ
Equil.
HX
.2438M
-x
.2438-x
H3O+ 0
+x
x
X0
+x
x
Ka = [ X-] [H3O+]
[HX]
•
1.81 x 10-5 = x2
.2438
x = .00210 = [H3O+]
pH = 2.68
• 5. What is the pOH, and Kb of the original
HX solution?
solution
• pH + pOH = 14
• pOH = 14 – 2.68
• pOH = 11.32
• Ka x Kb = 1.0 x 10-14
• Kb = 1.0 x 10-14
1.81x 10-5
Kb = 5.52 x 10-10
Titration Terminology
Standard (Acid or Base)
A solution of known concentration used to identify the unknown concentration of
another acid or base.
Indicator
A weak acid or weak base that changes color at a specific pH or acid
concentration.
Endpoint
The point in the titration when the color of the indicator changes and the addition
of the titrant should stop. (Ideally, this will be when the moles of base added
equals the moles of acid originally present in the solution.
Equivalence Point
The point in the titration when the moles of base added equals the moles of acid
originally present based on the chemical equation and stoichiometry.
Color Change Ranges For Some
Common Indicators
Typical Titration Curves
Strong Base into Strong Acid
Strong Base into Weak Acid
Weak Base into Strong Acid
Strong Base into Diprotic Acid
Strong Acid into Strong Base
Acid/ Base Conjugate Pairs
• The reversible reaction contains two acids and
two bases. We think of them in pairs, called
conjugate pairs.
• When the acid, HA, loses a proton it forms a
base, A-. When the base, A-, accepts a proton
back again, it obviously refoms the acid, HA.
These two are a conjugate pair.
• Members of a conjugate pair differ from each
other by the presence or absence of the
transferable hydrogen ion.
Another Conjugate Pair
• Ammonia in Water:
Part 2 Titration Race
• Each group is provided with 2 burets, each filled with
water.
• Each group is provided with 100mL of standardized
0.175M NaOH and 100mL of HCl of unknown Molarity.
• Each group is provided Phenolphthalein indicator and a
pH probe.
• Each group is required to complete 2 titrations to
determine the Molarity of the HCl and the Standard
deviation.
• Scoring will be ranked by accuracy/precision and ties will
be broken by time.
Standard Deviation