CHM 152L • Lab 6
Detective, an unidentified weak acid has been discovered in the chemistry lab. Your mission: perform titrations, gather evidence, and determine its true identity.
Think of titration like slowly filling a cup to the brim. You have a solution with an unknown concentration (the analyte) in a flask, and you slowly add a solution of known concentration (the titrant) from a buret until the reaction is exactly complete.
That exact moment when all the acid has reacted with all the base is called the equivalence point. At the equivalence point, moles of acid = moles of base.
But how do you see the equivalence point? You use an indicator. In this lab, you will use phenolphthalein, which is colorless in acidic solutions and turns pink when the solution becomes basic. The moment you see a persistent pink color that does not fade, you have reached the endpoint.
In your own words, explain what happens at the equivalence point of an acid base titration and how phenolphthalein helps you detect it.
Potassium Hydrogen Phthalate (KHP) is a primary standard, meaning its purity and molar mass are precisely known. That makes it a reliable witness for determining the concentration of your NaOH solution.
KHP reacts with NaOH in a simple 1 : 1 ratio:
Because the ratio is 1 : 1, moles of NaOH at the equivalence point equals moles of KHP. To find moles of KHP, use: moles = mass ÷ molar mass.
📋 Scene Evidence:
Mass of KHP: 0.511 g
Molar Mass of KHP: 204.22 g/mol
moles = 0.511 g ÷ 204.22 g/mol = ?
Express your answer in scientific notation.
Add NaOH slowly to the KHP solution. Watch the pH rise and the indicator change. When phenolphthalein turns pink and stays pink, you have found the equivalence point.
| Volume NaOH: | 0.00 mL |
| pH: | 0.00 |
| Equiv. Point: | ? |
| [NaOH]: | ? |
Tip: Use small increments (+0.5 mL) as you approach the steep part of the curve for a precise equivalence point.
Now that you found the equivalence point, you can determine the NaOH concentration. At the equivalence point:
moles NaOH = moles KHP (1 : 1 ratio)
And concentration = moles ÷ volume (in liters). The volume is the equivalence volume you recorded from Titration 1.
Moles of KHP = 2.502 × 10⁻³ mol
Equivalence Volume = complete Titration 1 first
Calculate the molar concentration of your NaOH solution.
concentration (M) = moles ÷ volume in liters
This equation is the key tool for solving this mystery. It connects pH, pKa, and the ratio of conjugate base to weak acid:
pH = pKa + log([A⁻] / [HA])
Here is the critical insight: at the half equivalence point, exactly half the weak acid has been neutralized. That means the amount of weak acid remaining [HA] equals the amount of conjugate base formed [A⁻].
When [HA] = [A⁻], the ratio is 1, and log(1) = 0. So the equation simplifies to:
pH = pKa
This means you can read pKa directly from the titration curve at the half equivalence point. Then calculate Ka = 10−pKa.
One of these five acids was found at the crime scene. After Titration 2, you will match your experimental Ka to identify the culprit.
Using the Henderson Hasselbalch equation, explain step by step why pH = pKa at the half equivalence point.
📋 Evidence from the scene:
Mass of unknown acid: 0.253 g (weighed on the analytical balance)
Dissolved in: 50 mL distilled water
Your standardized NaOH: 0.0991 M
Identity: UNKNOWN
Your investigation plan:
1. Titrate the unknown acid with NaOH and build the pH curve.
2. Identify the equivalence point (steep pH jump, indicator turns pink).
3. Find the half equivalence volume (half the equivalence volume).
4. Read the pH at the half equivalence point from your curve. That pH equals the pKa.
5. Calculate Ka from pKa and compare to the suspect lineup.
Titrate the unknown acid with your standardized NaOH. Watch the curve carefully. After you find the equivalence point, you will analyze the data on the next slide.
| Volume NaOH: | 0.00 mL |
| pH: | 0.00 |
| Equiv. Point: | ? |
| Half Equiv.: | ? |
After the equivalence point is found, examine your curve at the half equivalence point. What is the pH there?
Your Titration 2 results:
Equivalence Point: complete Titration 2 first
Half Equivalence Point: complete Titration 2 first
Look at your titration curve. Find the pH where the half equivalence line crosses your data. Remember: at the half equivalence point, pH = pKa.
What is the pH at the half equivalence point? (This equals the pKa of the unknown acid.)
Using your pKa, calculate Ka = 10−pKa
Compare your experimental Ka to the suspects. Click the acid you believe is the culprit.
The unknown acid is Benzoic Acid (C6H5COOH)
Molar Mass: 122.12 g/mol
Now that you identified the acid (MW = 122.12 g/mol), calculate the moles in the 0.253 g sample.
moles = mass ÷ MW = 0.253 g ÷ 122.12 g/mol
Before closing the case, demonstrate your mastery of acid base concepts.
Strong bases dissociate completely. For NaOH: [OH⁻] = [NaOH]. Then pOH = −log[OH⁻] and pH = 14 − pOH.
Conjugate base Kb: For any conjugate acid base pair, Kw = Ka × Kb. Since Kw = 1.0 × 10⁻¹⁴, you can find Kb = Kw ÷ Ka.
What is the pH of a 0.01 M NaOH solution?
Given Ka of acetic acid = 1.8 × 10⁻⁵, calculate Kb of the acetate ion (CH3COO⁻).
Kb = Kw ÷ Ka = (1.0 × 10⁻¹⁴) ÷ (1.8 × 10⁻⁵)
Your detective performance report
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