Why is estimation of sodium hydroxide more accurate with oxalic acid rather than with sulfuric acid or any other strong acid?
3
-
$\begingroup$ Who told you that ? $\endgroup$– MauriceJan 15, 2020 at 10:21
-
2$\begingroup$ Consider the possibility that as oxalic acid is a solid it can be weighed out with greater accuracy than an acidic solution $\endgroup$– WaylanderJan 15, 2020 at 10:47
-
1$\begingroup$ You are right, Waylander ! Thank you ! $\endgroup$– MauriceJan 15, 2020 at 11:32
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
4
There are a few reasons. Some of the most important are:
-Oxalic acid dihydrate isn't hygroscopic (it doesn't absorb water from the atmosphere, whereas sulfuric acid does). If a substance can absorb water from the atmosphere, then when you weigh it out, you won't be able to accurately calculate the number of moles used because you don't know how much is water and how much is your substance. This won't be the case with oxalic acid dihydrate.
It's chemically stable when stored.
As Waylander said (in his comment elsewhere), it's a solid so is easily weighed out.
It has a moderately high molecular mass of $\pu{126 g/mol}$ (as the dihydrate) so any error in the mass weighed corresponds to a relatively small error in the calculated amount in $\pu{moles}$.
As a result, oxalic acid has become a primary standard: So you can use it to make a solution of accurately known concentration to determine the concentration of a solution of sodium hydroxide (by titration).
-
2$\begingroup$ I'm curious: since you are using "number of moles" for the amount of substance, then why don't you use a number of kilograms in place of "mass"?:) $\endgroup$– andselisk ♦Jan 15, 2020 at 15:59
-
$\begingroup$ Well, because I was specifically referring to calculating the number of moles, which I think is clearer than referring to calculating the amount of substance, whereas "mass weighed" is unambiguous. $\endgroup$– atbmJan 15, 2020 at 19:00
-
-
$\begingroup$ Well I edited your answer to make it better as andselisk suggested. If you are not agreeing with the corrections, you have rights to change it since you are the author. I'd also like to include the fact that before estimating $\ce{NaOH}$ with $\ce{H2SO4}$, you need to standardize the concentration of $\ce{H2SO4}$ solution using primary standard because concentration of conc. $\ce{H2SO4}$ is uncertain. Also, dilute $\ce{H2SO4}$ is not hygroscopic as its concentration version. :-) $\endgroup$ Jan 16, 2020 at 17:08