The Best Way to Calculate pH

[Vanguard23]

---

Members don't see this ad.

I'm looking through my study materials(EK, TPR and TBR) and am I correct in assuming that TBR's method of calculating pH is the "superior" method? They use a "shortcut" calculation that is derived from the Ka value. Basically:

pH = 1/2pka -1/2log[HA]

It seems to work very well(for acids with a pKa between 2-12(I don't have it in front of me, but that could be incorrect)) and I can calculate pH with just the pKa and [HA] OR the Ka and [HA].
The other two guides recommend using the Henderson Hasselbach derivation, which seems more timely.

Opinions?

 

[RogueUnicorn]

i honestly never memorized shortcuts like this because i feel it uses unnecessary brainspace to save me 2 seconds

 

[BerkReviewTeach]

I'm looking through my study materials(EK, TPR and TBR) and am I correct in assuming that TBR's method of calculating pH is the "superior" method? They use a "shortcut" calculation that is derived from the Ka value. Basically:

pH = 1/2pka -1/2log[HA]

It seems to work very well(for acids with a pKa between 2-12(I don't have it in front of me, but that could be incorrect)) and I can calculate pH with just the pKa and [HA] OR the Ka and [HA].
The other two guides recommend using the Henderson Hasselbach derivation, which seems more timely.

Opinions?

That is correct and an extremely useful trick for getting the pH fast. It is basically the ice box method where instead of going step-by-step and showing your work, you start at the last step. In the current book (2009 version?), there is a more updated version of that equation which is easier to remember. It was mentioned in another post in the Q and A section.

BTW, the Henederson-Hasselbalch equation will NOT work for calculating the pH of a weak acid solution. It only applies to buffers (mixtures of HA and A- in roughly equal concentration).

i honestly never memorized shortcuts like this because i feel it uses unnecessary brainspace to save me 2 seconds

I agree that some tricks are a waste of time to practice, because you don't gain much. But this is one that is incredibly useful and would save thirty seconds to a minute for you.

 

[Vanguard23]

That is correct and an extremely useful trick for getting the pH fast. It is basically the ice box method where instead of going step-by-step and showing your work, you start at the last step. In the current book (2009 version?), there is a more updated version of that equation which is easier to remember. It was mentioned in another post in the Q and A section.
.

Out of curiosity, what is the updated method of doing so?

 

[Vanguard23]

Sorry to bump, but I'm really curious as to TBR's new method for calculating the pH of weak acids.
Thanks!

 

[RogueUnicorn]

i wonder if it's pH= 1/2pKa + 1/2p[HA]

 

[Vanguard23]

i wonder if it's pH= 1/2pKa + 1/2p[HA]

is that one published by another company?

 

[HopefulOncoDoc]

Hey Vanguard this is all from BerkReviewTeach,

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.

 

[Vanguard23]

Thank you very much. Yeah, I noticed the similarity between taking the average of the pKa and the pH of a weak acid as it applies to titrations.

 

[RogueUnicorn]

is that one published by another company?

no idea

 

[luctoretemergo]

no idea

Sorry to bump an old thread but, could some discrimimate when to use the phweak acid equation? perhaps with an example of the normal way versus use of this method?

 

[Don Draper]

Sorry to bump an old thread but, could some discrimimate when to use the phweak acid equation? perhaps with an example of the normal way versus use of this method?

There are only about 7 strong acids you need to know for the MCAT. If it isn't a strong acid, then it will be a weak one.

Strong acid is complete dissociation, so you just use the normal pH = [H3O]... for the weak acids you use the other equation.

from my notes:

• **Strong acids (w/ pKa)
  • Hydrochloric (HCl) -7
  • Hydrobromic (HBr) -7
  • Hydroiodic (HI) -9
  • Sulfuric acid (H2SO4) -9 (pKa1)
  • Nitric acid (HNO3) -2
  • Perchloric HClO4 -10
• Weak acid only partial dissoc, hence small Ka (< 1)
  • they define weak as pKa 0-14
  • Trichloroacetic acid Cl3CCO2H 0.64
  • Dichloroacetic acid Cl2HCCO2H 1.27
  • Sulfurous acid H2SO3 1.82 (pKa1)
  • Chlorous acid HClO2 1.9
  • Chloroacetic acid ClH2CO2H 2.82
  • Hydrofluoric acid HF 3.15
  • Nitrous acid HNO2 3.41
  • Formic acid HCO2H 3.74
  • Acetic acid H3CCO2H 4.74
  • Carbonic acid H2CO3 6.36 (pKa1)
  • 4-nitrophenol 4-O2NC6H4OH 7.15
  • Hypochlorous acid HClO 7.46
  • Hypobromous acid HBrO 8.72
  • Ammonium NH4+ 9.26
  • Hydrogen Cyanide HCN 9.32
  • Hypoiodous acid 10.66
• very weak acid, pKa > 14