Question

If I don't know the concentration of the `"HCl"` that I am titrating with `"NaOH"`, can I still use the `"NaOH"` to titrate an unknown acid and determine its molar mass?

Answer 1

You need at least one solution of known concentration, called your primary standard, if you want to use a solution of unknown concentration (your secondary standard) as a titrant into a solution of unknown concentration to determine its concentration.

In short, an example process for the molar mass of an unknown acid would be:

1. Prepare a solution of known concentration, such as `"1.0 M HCl"`.
2. Prepare a solution of "unknown" concentration. Often it is `"NaOH"`, which is hygroscopic and changes concentration over time by absorbing water.
3. Titrate the `"HCl"` into the `"NaOH"` to determine the concentration of the `"NaOH"`.
4. Titrate the `"NaOH"` into a solution of unknown weak acid, `"H"_n"A"`, that YOU have prepared (and thus know the mass you weighed).
5. Use the now-known concentration of `"NaOH"` to find the equivalent mols of `"H"_n"A"`.
6. Divide the mols you just found by `n`, the number of protons in `"H"_n"A"`, if you know already that the acid has more than one proton.
7. You can then use the mols of `"H"_n"A"` and its mass to find its molar mass.

Alternatively, instead of what has been stated for step `6`, you could just NOT do step `6`, and step `7` becomes the determination of the equivalent molar mass.

Then, you would multiply by the number of protons you know the acid has to determine its actual molar mass. That accomplishes the same thing.