Ka, pKa & the pH of Weak Acids (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about Ka, pKa, and the pH of weak acids.

Ka measures how far a weak acid dissociates, and pKa is simply the log form of that same value — the same relationship the concentration of hydrogen ions has to pH — which is why finding the pH of a weak acid from Ka, and finding Ka from a measured pH, are really the same calculation run in opposite directions.

A weak acid only partially dissociates in solution, and the equilibrium constant for that dissociation is called the acid dissociation constant, Ka — the higher the Ka, the more the acid dissociates, and the stronger it is. Because Ka values are awkward, very small numbers, it's easier to work with pKa, the negative log of Ka, in exactly the same way pH is the negative log of the concentration of hydrogen ions. That relationship works in both directions: Ka lets you calculate the pH of a weak acid from its concentration, and a measured pH can be used to calculate Ka.

This video starts with the acid dissociation constant itself and how it's used to find the pH of a weak acid, then covers pKa as the more workable form of Ka, before finishing with how Ka is found experimentally from a pH titration curve.

A weak acid only partially dissociates in aqueous solution, so an equilibrium is established between the undissociated acid and its ions. The equilibrium constant for that dissociation is called the acid dissociation constant, Ka, and it has units of moles per decimetre cubed. Values of Ka are very small — for ethanoic acid, Ka is 1.74 times 10 to the power of minus 5 moles per decimetre cubed. The higher the value, the more dissociated, and so the stronger, the acid is. This is the constant that pKa is simply a more convenient way of expressing, and that the pH of a weak acid is calculated from.

Because one molecule of a weak acid, HA, dissociates into one H plus ion and one A minus ion, their equilibrium concentrations are equal, which simplifies the Ka expression to the concentration of hydrogen ions squared equals Ka multiplied by the acid concentration. Taking the square root gives the H plus concentration equal to the square root of Ka multiplied by the acid concentration Taking the negative log of that gives the pH directly: pH equals minus log of the square root of Ka multiplied by the acid concentration. This is the same H plus concentration-to-pH relationship used for strong acids — it just needs Ka to get to H plus concentration first, because a weak acid doesn't fully dissociate.

Because Ka values are very small and awkward to compare, it's easier to work with pKa, the negative log of Ka — exactly the same idea as converting H plus concentration into pH. Most weak acids have a pKa between 3 and 7, and because it's a log scale, a smaller pKa means a larger Ka, and so a stronger acid. pKa doesn't just make Ka easier to write down — it also turns out to be readable directly off a pH titration curve, which is how Ka is actually found experimentally.

To find the Ka of a weak acid experimentally, it's neutralised with a strong base while its pH is measured and plotted against the volume of base added, producing a pH titration curve. When exactly half the acid has been neutralised, the concentration of the remaining acid, HA, equals the concentration of its conjugate base, A minus. At that specific point, pKa equals pH. So reading the pH off the curve at half the volume needed for neutralisation gives pKa directly, and from there, Ka.

The symbol p just means minus log base 10 of a value — you don't need to write the 10, since log on its own means log base 10, while ln is used for a natural logarithm. That gives a useful shortcut: since Kw equals H plus concentration multiplied by OH minus concentration, taking pKw gives pKw equals pH plus pOH, which equals 14.00 at 298 kelvin.

Ka measures how far a weak acid dissociates, and its value is used to calculate the pH of that acid from its concentration. pKa is simply the negative log of Ka, put on the same easier-to-use scale as pH. And that relationship runs both ways: at half-neutralisation on a titration curve, pKa equals pH, which is how Ka is found experimentally in the first place.

On Save My Exams, you'll find exam specific revision, made by trusted examiners. Study with revision notes, practice with exam questions, and improve your grade by targeting your weak spots. Visit the links in the description.