Question

Calculate mass of oxalic acid (H2C2O4.2H2O) needed to prepare 250 ml ,1 M aqueous solution of oxalic acid . I got 15 g . Is my answer correct ?

Answer 1

That answer is, unfortunately, wrong. Instead of telling you the answer, I will give you some key clues to getting it correct yourself.

Explanation:

First off, do not fall into the trap of confusing one mole of acid with one mole of hydrogen ions in solution. You need to work with the whole molecular formula of the acid just like you would do with any other compound. In other words, the reaction

`"H"_2"C"_2"O"_4•2"H"_2"O"\rightarrow 2"H"_3"O"^+ +"C"_2"O"_4^{2-}`

is not relevant, only the original formula `"H"_2"C"_2"O"_4•2"H"_2"O"` is relevant to determining a one molar solution. Incidentally, a 1M solution of oxalic acid does not come anywhere near producing 2M (solvated) hydrogen ions. Oxalic acid in water is only a weak acid.

Second: Remember to include water of hydration in the formula, too, meaning the `2"H"_2"O"` part of the formula counts towards the mass of one mole, too. I need to be careful with that myself in the lab at work, or else I do not make up my pickling acid solutions (containing hydrochloric acid and iron chloride) correctly.

Good luck!

Answer 2

No.

But I expect the mass to be around `30 - "40 g"`, because around `"2 g"` is needed for a `"0.05 M"` solution in a `"200 mL"` flask, if I remember correctly.

Since we want `"250 mL"` of `"1 M H"_2"C"_2"O"_4(aq)`, we want

`("1.00 mol H"_2"C"_2"O"_4)/(cancel"1 L soln") xx 0.250 cancel"L soln" = "0.250 mols H"_2"C"_2"O"_4`

But for every one `"H"_2"C"_2"O"_4`, there is one `"H"_2"C"_2"O"_4cdot2"H"_2"O"`, so we need `"0.250 mols"` of specifically `"H"_2"C"_2"O"_4cdot2"H"_2"O"` that is added to some water and then filled up to the `"250 mL"` mark on a volumetric flask.

Therefore, the mass needed is:

`0.250 cancel("mols H"_2"C"_2"O"_4cdot2"H"_2"O") xx (??? "g")/(cancel("1 mol H"_2"C"_2"O"_4cdot2"H"_2"O"))`

`=` `???`

When I worked it out, it was on the lower end of the range between `30` and `40` `"g"`. Now work it out and convince yourself that the answer lies in this range.