Introduction to Titrations

Concentration tells us how much solute is present in a given amount of solution. Molarity (M) is the most common unit of concentration and is defined as:

$$M = \frac{\text{moles of solute}}{\text{liters of solution}}$$

A titration is a controlled chemical reaction used to determine the concentration of an unknown solution by reacting it with a known amount of another substance. In this lab, the base (NaOH) will be titrated against the acid (KHP).

KHP stands for potassium hydrogen phthalate, and its chemical formula is:   KHC₈H₄O₄

It is a solid, monoprotic acid, and serves as a primary standard because it is:

• Highly pure

• Stable

• Easily weighed

The actual reaction between KHP and sodium hydroxide (NaOH) is:

$${\text{KHC}}_8{\text{H}}_4{\text{O}}_4(aq)+\text{NaOH}(aq)\to {\text{KNaC}}_8{\text{H}}_4{\text{O}}_4(aq)+{\text{H}}_2\text{O}(l)$$

This reaction proceeds in a 1:1 molar ratio: one mole of KHP reacts with one mole of NaOH.

To visually detect when all the KHP has reacted, phenolphthalein is added to the solution. It is:

• Colorless in acidic solution

• Turns pale pink in basic solution

The moment the solution turns and remains faint pink, the endpoint of the titration has been reached. The equivalence point is the theoretical point where the moles of base = moles of acid. The endpoint is the visible color change caused by the indicator. In a properly done titration, these two points occur very close together.

Example Calculation

Given:

• Mass of KHP = 0.650 g

• Volume of NaOH used = 31.00 mL (0.03100 L)

• Molar mass of KHP = 204.22 g/mol

Step 1: Moles of KHP

$$\text{mol KHP}=\frac{0.650\text{g}}{204.22\text{g/mol}}=0.00318\text{mol}$$

Step 2: Moles of NaOH (1:1 ratio)

$$\text{mol NaOH} = 0.00318 \, \text{mol}$$

Step 3: Molarity of NaOH

$$M = \frac{0.00318 \, \text{mol}}{0.03100 \, \text{L}} = 0.1026 \, \text{M}$$

Through this titration procedure, we determine the true molarity of NaOH by reacting it with a known quantity of KHP, using phenolphthalein to visually identify the endpoint. This experiment demonstrates the importance of stoichiometry, careful technique, and quantitative analysis in chemistry.