Henderson Hasselbalch Equation

Memorization Hack 💡

Protonated: Has a proton! HA contains an H in the name! This is acidic, as more acidic substances contain more H+. 

Deprotonated: No proton, does not contain an H in the name. This is basic which can be remembered as it only has one letter. "Basic!"  

Notes:

The Henderson Hasselbalch equation is pH= pKa + log([A-]/[HA]) 

• pH: this is a measurement of the acidity or basicity of a substance. 

• pKa: the negative logarithm of the acid dissociation constant (Ka). It measures an acid's strength. 

• [A-]: The concentration (molar) of basic molecules 

• [HA]: The concentration (molar) of acidic molecules

This equation can be modified to find missing information. For instance, if a question is asking to find the [A-], the rest of the equations can be plugged in and solved for the unknown. 

This can also be used to find the percentage of protonated or deprotonated molecules. To do this, you can solve for [A-]/[HA] by taking 10^ of the other side of the equation to cancel out the log. 

So, say you end up with: 

2=log([A-]/[HA]) 

It would become: 10^2 = [A-]/[HA] or 100 = [A-]/[HA]

This would mean that there were 100 A- parts per 1 HA part. 

To find the percentage: 

Put the one you are interested in the numerator (so either [A-] for deprotonated or [HA] for protonated) and the total amount of parts (so [A-] + [HA]) in the denominator. Then, you multiply this by 100. 

So, to find the percentage of deprotonated it would be: 

[A-]/[([A-] + [HA]) 

100/101 = .99 x 100 = 99% 

To find the percentage of protonated it would be: 

[HA] / ([A-] + [HA]) 

1/101 = .01 x 100 = 1%