Final answer:
The molar mass of the unknown diprotic acid is calculated by determining the moles of NaOH used in neutralization, establishing the moles of H2A based on the stoichiometry, and dividing the mass of the acid by the moles of acid. The approximate molar mass is found to be 394.97 g/mol.
Step-by-step explanation:
To calculate the approximate molar mass of the unknown diprotic acid (H2A), we first need to determine the amount of NaOH used to neutralize it. Since 31.81 ml (0.03181 L) of 0.109 M NaOH is required for neutralization, we can calculate the moles of NaOH used in the reaction using the formula:
moles of NaOH = (Volume in L) × (Molarity) = 0.03181 L × 0.109 M = 0.003469 moles NaOH
Since H2A is diprotic, each mole of H2A will react with 2 moles of NaOH. Therefore, the moles of H2A will be half the moles of NaOH:
moles of H2A = = 0.003469 moles NaOH / 2 = 0.0017345 moles H2A
To find the molar mass, we divide the mass of the acid sample by the moles of acid:
Molar Mass of H2A = (Mass in grams) / (Moles of acid) = 0.685 g / 0.0017345 moles = 394.97 g/mol
Thus, the approximate molar mass of the acid is 394.97 g/mol.