Acid Identification

Question

1. To identify an unknown diprotic acid, you dissolved 2.0 g of th e acid into 100 mL of water, 

titrated it with 0.2 M NaOH solution, and found the equivalence point after titrating 222.00 mL 

NaOH solution. What is this unknown acid?

Answer 1

Let the diprotic acid be H₂A (2 titratable hydrogens in a diprotic acid)

H₂A + 2NaOH ==> Na₂A + 2H₂O .. balanced equation for FULL titration of both hydrogens

moles of NaOH needed = 222.00 ml x 1 L / 1000 ml x 0.2 mol / L = 0.0444 mols NaOH needed

moles H₂A present = 0.0444 mols NaOH x 1 mol H₂A / 2 mols NaOH = 0.0222 mols H₂A

Molar mass H₂A = g H₂A / mols H₂A = 2.0 g / 0.0222 mols

Molar mass H₂A = 90.1 g / mole

As in your previous question, it is impossible to positively identify this unknown acid based only on the molar mass. But if you look for a common diprotic acid with a molar mass of around 90, you will find OXALIC ACID fits the bill. The formula for oxallic acid is C₂H₂O₄ and this has a molar mass of 90.03 g / mol.

Answer 2

From my understanding based on the information given, the reaction could be written:

NaOH(aq) + unknown acid(aq) <---> H2O + H3O+ or H+

.0444 mol of NaOH being added to reach equivalence point is the point at which equal amounts of reactants have been added. I find this because Molar, or M, is moles/Liter, or mol/L, so multiple .222L by 0.2M of NaOH determines moles of NaOH.

at the equivalence point, we assume moles of NaOH added equivalent to moles of unknown acid, so we take the grams of acid, 2g and divide by equivalents determined, 0.0444 moles, to find g/mol or the molar mass, 45.045g/mol. This in and of itself does not lead to the determination of what the unknown acid is but should enable you to do so if provided with adequate information. Also, the 100mL of water is another factor to take into account since the 2.0g of unknown acid was added in 100mL of water initially.

Beyond that, more information may be needed like a table to determine the unknown acid, which may be some monoprotic acid.

I say it may be monoprotic because in the information given, you say "the equivalence point" indicating there is only one equivalence point for this chemical species which coincides with what monoprotic acids display.

NaOH(aq) + HA(aq) <----> H2O + H+