why is KH2PO4 a acidic salt? (1 Viewer)

[anonymoushehe]

I know that KH2PO4 is made up of K+ which has no tendency to donate/accept a proton, but for H2PO4 -, its the weak conjugate base of the weak H3PO4 but also the weak conjugate acid of HPO42- and I thought that the reaction between H2PO4 - with water occurs to a greater extent than the reaction between HPO4 2- and water, so there is an overall excess of OH- but apparently its acidic??

 

[cheesynooby]

first ionisation of H3PO4 occurs to a relatively large extent - so the ability of H2PO4- to accept protons only occurs to a fairly small extent, that is, H2PO4 isn't too basic in nature
second ionisation of H2PO4 occurs to a smaller but still fairly large extent so overall it donates protons more than it accepts protons
this might not be too accurate btw idk what im talking about that well

 

[Socialism]

HOORAY someone answered ur question im sorry anonymoushehe

 

[wizzkids]

Yes, KH₂PO₄ is acidic, but not by much.
Here is another way to visualise the acidity/basicity of the various phosphate ions.
On the very acidic side we have phosphoric acid.
H₃PO₄. pK_a = 2.5
Start removing hydrogen ions and substituting relatively neutral potassium ions, and we construct compounds that are progressively less and less acid. I'm going to quote pK_a values, if you don't understand pK_a at this time, don't worry, just think about it as a description of how acidic a compound is when ionised in water at some standard concentration.
KH₂PO₄ pK_a = 6
K₂HPO₄ pK_a = 9
K₃PO₄ pK_a = 12