[galpinj]

Hello folks,

quick question on equivalence point. I was working through some problems and ran into a conceptual issue.

The question was: "When titrating the monoprotic acid HCl with NaOH of a known concentration, which of the following must be equal at the equivalence point?"

The two answers I narrowed it down to were:
A.Initial moles of HCl are equal to the moles of OH- added
B.Concentrations of [Na+] and [Cl-] are equal

A is correct

I was familiar with A, as I have read similar statements in my textbook; however, I couldn't explain why option B. is wrong. At equivalence point, an equal amount of OH- and H+ will be present. Obviously, the titrant was added as NaOH to the analyte (HCl). As such, wouldn't there be an equimolar amount of Cl- and Na+? Furthermore, given that they will be in the same solution (after the titrant has been added), they should have the same concentration.

Why is B incorrect?

[AWK]

Indicators (eg methyl orange) may contain some Na⁺ ions.
In precise electrochemical titrations very often some amount of salt (ie NaNO₃) may be added to obtain ionic strength 0.1.

[galpinj]

Indicators (eg methyl orange) may contain some Na⁺ ions.
In precise electrochemical titrations very often some amount of salt (ie NaNO₃) may be added to obtain ionic strength 0.1.

Ah, I see. Thanks!