What volume of a .20 M H3PO4 is needed to react with 100 mL of .1 M NaOH? H3PO4 + 3 NaOH --> Na3PO4 + 3 H2O

1 Answer
anor277
Apr 9, 2018

The premise of the question is INCORRECT....phosphoric acid is NOT a TRIACID...

Explanation:

Phosphoric acid meets a stoichiometric endpoint with TWO equiv of base....i.e.

#H_3PO_4(aq) + 2NaOH(aq) rarr Na_2HPO_4(aq) + 2H_2O(l)#

And so ....

#"moles of sodium hydroxide"-=100xx10^-3*Lxx0.10*mol*L^-1=0.01*mol#

...and therefore, there were HALF this molar quantity with respect to #H_3PO_4# ... and thus a volume of...

#(0.01*mol)/2xx1/(0.20*mol*L^-1)xx10^3*mL*L^-1=25.0*mL#

Note that at A level, and certainly at an undergraduate level, I would expect a punter to know the speciation of phosphoric acid in aqueous solution.... I think the round numbers obtained in the answer supports this interpretation...

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