[StonerPenguin]

My friend who is *supposedly* quite good at Chemistry helped me out the other day and it doesn't seem right to me;

"Hydrochloric Acid (HCl) is a relatively strong acid. Imagine that HCl is added to a buffer solution containing the weak acid phosphoric acid (H3PO4) and a salt (Na2HPO4) Answer the following questions."

"A. Will HCl react with the weak acid (H3PO4), or will it react with the salt (Na2HPO4)?"
>> Friend's answer; The hydrochloric acid will react with the weak acid, phosphoric acid."

"B. One product of this reaction will be a new salt (NaCl) What will the other be?"
>> Friend's answer; Na2HPO4 + HCl --> NaCl + NaH2PO4 So it's NaH2PO4

The second one doesn't seem right to me, the reaction he put is with the HCl and the salt, not the phosphoric acid. Am I missing something here? He's wrong isn't he?

Thank you for any help

[Jorriss]

Does part A seem fine to you also? Because it's incorrect. Look at that first.

[cliverlong]

My friend who is *supposedly* quite good at Chemistry helped me out the other day and it doesn't seem right to me;

"Hydrochloric Acid (HCl) is a relatively strong acid. Imagine that HCl is added to a buffer solution containing the weak acid phosphoric acid (H3PO4) and a salt (Na2HPO4) Answer the following questions."

"A. Will HCl react with the weak acid (H3PO4), or will it react with the salt (Na2HPO4)?"
>> Friend's answer; The hydrochloric acid will react with the weak acid, phosphoric acid."

For number 1 I would suggest the following

since hydrochloric acid is strong it is (almost?) 100% disociated in water into hydrogen and chloride ions

HCl(aq)  H⁺ + Cl^-

Phosphorc acid is triprotic, and as a weak acid its disociations are not 100%

H₃PO₄  H⁺ + H₂PO4^-

(from http://www2.ucdsb.on.ca/tiss/stretton/Database/polyprotic_acids.htm)

                  Ka                           pKa
1st      7.1 x 10^-3      2.15

2nd     6.3 x 10^-8    7.20

3rd       4.5 x 10^-13     12.35

So by Le Chatelier's principle, HCl(aq) will not react with phosphoric acid but it will provide so much H⁺ to force almost all of the phosphoric acid to undissociated molecules. You can think of the H₂PO₄^- acting as a strong base to soak up the H⁺