Relationship between pKa and pH

[Chloe Fuson]

I know that the lower the pKa value, the stronger the acid, but what exactly is the relationship between pKa and pH when it comes to determining the the charge of a predominant species in a reaction? (for example, like question 10 on the week 2 achieve assignment)

[Morgan Oey-3H]

For number 9 on the achieve week #2, you can compare the two values of pH and pKa to determine the more predominant species. If pH is above the pKa, the deprotonated species of A- will become more predominant in the solution. If pH is lower than pKa, the neutral species of HA is more predominant. pH is a measure of the acidity, and in return, pKa is a measure of the deprotonated acid (how much of the acid is deprotonated)

[705573518]

For number 9 on the achieve week #2, you can compare the two values of pH and pKa to determine the more predominant species. If pH is above the pKa, the deprotonated species of A- will become more predominant in the solution. If pH is lower than pKa, the neutral species of HA is more predominant. pH is a measure of the acidity, and in return, pKa is a measure of the deprotonated acid (how much of the acid is deprotonated)

I was a little confused about this too. We started going over these types of problems in discussion today and my TA said we will start this material in class this week in case anyone else was alarmed about not having seen this yet in class. :)

[Jenny Han 3F]

When pH is greater than pKa, the deprotonated, conjugate base form is more present. If the pH is less than pKa, the protonated, conjugate acid form is present. I'm not sure if this is exactly right, but the way I remember it is that a lower pH means more [H+] that can protonate and a higher pH means less H+ for protonation.

[Sophia Zhao]

A good way to remember the relationship is to associate Ka with [A-] (there's an A in Ka) and pH with [HA] (there's an H in pH). When pKa>pH, you flip the relationship with A- and HA: [A-]<[HA]. HA is uncharged so there is a greater number of uncharged species, so the solution is neutral. When pKa<pH, you flip the relationship with A- and HA: [A-]>[HA]. A- is charged so there is a greater number of charged species, so the solution is protonated.