In lab, you're having fun along with buffers based onto a 0.1 M solution of sodium phosphate at pH7.0.  Into this system, there are three of pK_As as:  12.7, 7.21 and 2.12.

When you used this buffer to execute a separation of amino acids onto the Dowex (cation-exchange; which is, negatively-charged beads) chromatography column which you are using this week into lab, what sequence would you predict the amino acids Histidine, Glutamine, Lysine and Alanine to elute within?

A cation-exchange column keeps positively-charged species, and the most highly-charged species must be retained longest. At pH7, alanine and glutamine both must be fundamentally uncharged, they will elute primary (into the wash), mutually. Histidine will have a partial positive charge at pH7; this will be weakly retained and will elute well before lysine that must be strongly charged at such pH.