How to Calculate Molar Mass of a Diprotic Acid?

The molar mass of a diprotic acid can be calculated through titration with a base of known concentration, typically NaOH, using the following method:

Titration Method for Determining Molar Mass of a Diprotic Acid

The key to determining the molar mass of a diprotic acid lies in a precise titration process. This involves reacting a known mass of the acid with a solution of sodium hydroxide (NaOH) of known concentration until the reaction reaches the first equivalence point. According to our reference, the process involves these key steps:

1. Sample Preparation: Accurately weigh a known mass of the diprotic acid. This will give you the acid's mass in grams.

2. Titration: Titrate the weighed acid sample with a standardized NaOH solution (meaning the concentration of NaOH is known). Monitor the titration, often using a pH meter or an indicator, to precisely determine the volume of NaOH required to reach the first equivalence point. This is where the first proton of the diprotic acid has completely reacted with the hydroxide ions from the NaOH.

3. Moles of NaOH Calculation: Calculate the moles of NaOH used in the reaction. Use the following formula:

   • Moles of NaOH = Volume of NaOH (in Liters) × Molarity of NaOH (mol/L)

4. Moles of Diprotic Acid Calculation: Since diprotic acids have two protons, the first equivalence point indicates that moles of base equals moles of acid. Therefore:

   • Moles of Diprotic Acid = Moles of NaOH at first equivalence point

5. Molar Mass Calculation: The molar mass can now be found using the formula:

   Molar Mass (g/mol) = Mass of Diprotic Acid (grams) / Moles of Diprotic Acid (moles)

Example:

Let's say:

• Mass of Diprotic Acid: 0.250 grams
• Concentration of NaOH: 0.100 M
• Volume of NaOH needed to reach first equivalence point: 25.0 mL (0.025 L)

Here's how to calculate the molar mass:

1. Moles of NaOH: 0.025 L * 0.100 mol/L = 0.0025 moles
2. Moles of Diprotic Acid: 0.0025 moles
3. Molar Mass of Diprotic Acid: 0.250 g / 0.0025 mol = 100 g/mol

In this example, the molar mass of the diprotic acid is 100 g/mol.

Key Takeaways:

• This method relies on accurately weighing the diprotic acid and precisely measuring the volume of NaOH titrant.
• The first equivalence point is crucial. It is where one mole of NaOH reacts with one mole of the diprotic acid.
• The molar mass (molecular weight) is expressed in grams per mole (g/mol).

By following these steps, you can accurately determine the molar mass of a diprotic acid using titration.