How To Calculate Percentage Purity In Titration

How to Calculate Percentage Purity in Titration

Titration is a crucial analytical technique used to determine the concentration of a substance by reacting it with a solution of known concentration. A common application is determining the percentage purity of a sample. This involves calculating the actual amount of the desired compound present in a sample, taking into account any impurities. This guide will walk you through the process of calculating percentage purity using titration data.

Understanding the Fundamentals

Before diving into calculations, let's solidify our understanding of the key concepts:

What is Titration?

Titration is a quantitative chemical analysis method where a solution of known concentration (the titrant) is gradually added to a solution of unknown concentration (the analyte) until the reaction between them is complete. The point of completion, called the equivalence point, is usually indicated by a color change using an indicator.

Moles and Molarity

Understanding moles and molarity is fundamental to titration calculations.

• Moles: The amount of substance, measured in moles (mol). One mole contains Avogadro's number (6.022 x 10²³) of particles.

• Molarity (M): The concentration of a solution, expressed as moles of solute per liter of solution (mol/L).

Stoichiometry

Stoichiometry deals with the quantitative relationships between reactants and products in a chemical reaction. The balanced chemical equation is essential for determining the mole ratios between the titrant and the analyte.

Step-by-Step Calculation of Percentage Purity

Let's illustrate the process with an example. Suppose you're determining the percentage purity of a sample of sodium hydroxide (NaOH) using a standardized solution of hydrochloric acid (HCl).

1. Balanced Chemical Equation:

First, write down the balanced chemical equation for the reaction between NaOH and HCl:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

This equation shows a 1:1 mole ratio between NaOH and HCl.

2. Data Collection:

You'll need the following data from your titration experiment:

• Mass of impure NaOH sample: Let's say it's 2.50 g.
• Molarity of HCl titrant: Let's assume it's 0.100 M (mol/L).
• Volume of HCl used: Let's say 25.0 mL (0.0250 L) was required to reach the equivalence point.

3. Moles of HCl:

Calculate the moles of HCl used in the titration using the molarity and volume:

Moles of HCl = Molarity × Volume = 0.100 mol/L × 0.0250 L = 0.00250 mol

4. Moles of NaOH:

Using the stoichiometry from the balanced equation (1:1 mole ratio), the moles of NaOH in the sample are equal to the moles of HCl used:

Moles of NaOH = 0.00250 mol

5. Mass of Pure NaOH:

Calculate the mass of pure NaOH using its molar mass (approximately 40.00 g/mol):

Mass of NaOH = Moles × Molar Mass = 0.00250 mol × 40.00 g/mol = 1.00 g

6. Percentage Purity:

Finally, calculate the percentage purity of the NaOH sample:

Percentage Purity = [(Mass of pure NaOH / Mass of impure NaOH sample) × 100]% = (1.00 g / 2.50 g) × 100% = 40.0%

Important Considerations

• Accuracy: The accuracy of your percentage purity calculation relies heavily on the accuracy of your experimental measurements, particularly the molarity of the titrant and the volume used.
• Indicators: The choice of indicator is crucial for accurately determining the equivalence point.
• Error Analysis: Understanding and accounting for potential sources of error (e.g., systematic and random errors) in the experiment is essential for reliable results.

By following these steps and understanding the underlying principles, you can confidently calculate the percentage purity of a substance using titration data. Remember to always use appropriate safety precautions when performing titrations.