Acid Identification

Question

1. You have a package of chemical material to help with unknown chemical identification. The only 

known information about this package is for monoprotic acid. You dissolved 1.0 g of the acid into 

100 mL of water, titrated it with a 0.1 M NaOH solution, and found the equivalence point after 

titrating 118.4 mL NaOH solution. What is this unknown acid? 

 

Answer 1

In an attempt to identify the unknown acid, we will want to try to find the molar mass of this acid. To do so, we will determine the moles of acid present in the 1.0 g that were used. Caveat: Even after doing this, it will be impossible to identify for certain the unknown, unless we have additional information. This is a poorly conceived question.

Let HA be the monoprotic acid.

HA + NaOH ==> NaA + H2O .. balanced equation for neutralization

moles of NaOH used = moles HA present = 118.4 ml x 1 L / 1000 ml x 0.1 mol / L = 0.01184 moles HA

molar mass of HA = g / moles = 1.0 g / 0.01184 moles = 84.5 g / mole

Now, all you have to do is find a monoprotic acid (an acid with 1 hydrogen) with this molar mass. There are too many monoprotic acids to go through, so try to find one with a molar mass of around 85. HBr might be possible but it has a molar mass of only around 81 g / mol.