Question

Determine the concentration of the acid.?

Answer 1

With all these problems a stoichiometrically balanced equation is a prerequisite....

Explanation:

And it is a fact that phosphoric acid is a DIACID in aqueous solution....and so we interrogate the reaction...

`H_3PO_4(aq) + 2NaOH(aq) rarr Na_2^+HPO_4^(-)+2H_2O`

And we use the relation that `"concentration"="moles of stuff"/"volume of stuff"`..

...and so `"moles of stuff"="concentration"xx"volume"`

`n_"NaOH"=42.5*mLxx10^-3*L*mL^-1xx0.90*mol*L^-1=0.03825*mol`...

But given the stoichiometry, there was HALF this molar quantity with respect to `H_3PO_4` in the initial `20*mL` volume...so...

`[H_3PO_4]=(1/2xx0.03825*mol)/(20*mLxx10^-3*L*mL^-1)=0.9563*mol*L^-1` with respect to the ACID....

Note that it is a fact that phosphoric acid is HERE a diacid, not a triacid...this is something you simply need to know....