Calculating pH of Weak Acids

[SunChip]

Calculating the pH of weak reagents seems complicated. Is there a mnemonic for this? How do I solve pH of 1.00 M HF with pKa = 3.32?

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[HopefulOncoDoc]

Calculating the pH of weak reagents seems complicated. Is there a mnemonic for this? How do I solve pH of 1.00 M HF with pKa = 3.32?

There is a shortcut to calculating the pH of weak reagents using;

pH = 1/2pKa - 1/2log[HA] but it only works when the concentration is greater than the Ka and if the pKa falls between 2 & 12. Use log[HA] as if you were solving for a strong acid. Hope this helps.

pH = 1/2(3.32) - 1/2log(1) = 1.66

 

[SunChip]

Thanks so much! 🙂

 

[Vihsadas]

You can solve the "Log[HA]" part of this problem without a calculator using the pH trick outlined in the link in my signature. Good luck!

 

[BerkReviewTeach]

There is a shortcut to calculating the pH of weak reagents using;

pH = 1/2pKa - 1/2log[HA] but it only works when the concentration is greater than the Ka and if the pKa falls between 2 & 12. Use log[HA] as if you were solving for a strong acid. Hope this helps.

pH = 1/2(3.32) - 1/2log(1) = 1.66

You must have a slightly older edition of the general chemistry books. There is an updated version of this shortcut equation (Equation 4.15 in the older BR books) in the most recent books.

It's basically the same derivation, but it adds two additional steps where you factor out the half and note that the -log[HA] term is what the pH would be if it were strong. The result is that you average the pKa and the pHif the weak acid fully dissociated.

It reads:

• 
  pHweak acid = (pKa + pHif the weak acid fully dissociated)/2

The restrictions you listed are still applicable, as shown on page 258 of the older General Chemistry book 1, but now there is also a titration curve explanation that helps make it easier to visualize. The new version of the equation is meant to be a just a little easier to remember.

 

[hvairline]

pka=3.32 <=> Ka=10^-3.32 but Ka=x^2/1-x where x=[h3O+] and according to ostwald's principle 1-x=1 since Ka/c<<0.01 so x^2=10^-3.32 and x=10^-1.66
and ph=-logx so ph=1.66 but yeah the other way is a lot easier