pH and pKa relation for determining predominant species

[Yiyi Chen 1D]

Can someone summarize how we can know whether the predominant species present at a certain pH is neutral or charged (like in questions 9 and 10 in the week 2 achieve hw) based on the pKa?

[Alyssa_Miao_3B]

To answer questions like the ones on the Achieve homework, you just need to memorize the general rules for both acids and bases. For acids, when pH > pKa, the charged, deprotonated species (A-) will be predominant. When pH < pKa, the neutral, protonated species of HA will be predominant. For bases, when pH > pKa, the neutral, deprotonated species (B) will be predominant. When pH < pKa, the charged, protonated species (BH+) will be predominant. They are essentially the opposites of each other. I think once you memorize these, you should be able to easily identify the predominant species. Hope this helps!