Histidine has ionizable groups with pKa values of 1.8, 6.0 and 9.2. (These correspond to the COOH, imidazole, and –NH3 groups respectively). You have 100 mL of a 0.1 M solution of histidine at pH 5.40. You then add 40 mL of 0.1 N HCl. What is the resulting pH of the solution?

I'm learning about the periodic tables and what each subject's configuration is. for example, hydrogen is 1s^1, but i don't understand how you get that. can someone me understand how to figure out how to figure this out? sorry if the question makes no sense, but it would really a lot if you could me understand! you so much if you can!