common ion effect

[afatasstank]

Adding HCl or H2SO4 to a mixture of H2PO4-/HPO42- would cause a common ion effect. What is the common ion, and what would be the overall observation? please help.

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[unique135]

H2PO4- <====equilibrium====> HPO4-2 + H+

HCl -------------------> Cl + H+


HCl dissociates completely producing extra H+ in the solution of H2PO4-/HPO4-2. This solution before was already at equilibrium. Above that, adding extra H+ ions would change its equilibrium and degree of ionization (basically it would be reduced). Applying Le Chatelier's principle, the equilibrium will shift towards left to acheive new equilibrium with different concentrations.

H+ will be common ion since it is the one that is affecting equilibrium.

 

[afatasstank]

Wow, thanks!!