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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: senna on April 04, 2005, 01:58:37 AM

Title: Acid and base Strength
Post by: senna on April 04, 2005, 01:58:37 AM

I have a question that asks to list the acid strength in order of increacing acid strength.  The order that I have come up with is NH4, HCN, HPO4 2-, H3O+, HF and finally HCL.  

I feel a little confused as to where the HPO4 2- falls into the line up. It is also listed in the next problem that asks to list the bases in increasing strength. The order that I came up for the bases is HPO4 2-, CN-, Cl-, F-, NH3, OH-.  

I am confused as to how HPO4 2- can have a ka and kb value and how these numbers relate.  

??Do the order of these acids and bases appear correct?  

Thank you for your response!

Title: Re:Acid and base Strength
Post by: Borek on April 04, 2005, 04:35:08 AM

Quote

I am confused as to how HPO4 2- can have a ka and kb value and how these numbers relate.

Very basic fact. You didn't pay attention at school ;)

Let's start with a weak acid HA

HA = H+ + A-

Ka = [H+][A-]/[HA]

What about conjugate base A-? It can react with water:

A- + H2O = HA + OH-

Kb = [HA][OH-]/[A-]

(water concentration moved to Kb by convention)

Multiplying:

Ka * Kb = [H+][A-]/[HA] * [HA][OH-]/[A-]

[HA] and [A-] cancels out:

Ka * Kb = [H+][OH-]

so

Ka * Kb = Kw

Title: Re:Acid and base Strength
Post by: kkhchemistboy on April 07, 2005, 12:20:53 PM

Hi there,

You are contradicting yourself on the relative strengths of the acids and strengths of the bases...I would look up the term conjugate acid.....that should clear up some things.

Chemboy