For he says to Moses (tōi Mōusei gar legei). He has an Old Testament illustration of God’s election in the case of Pharaoh (Exo 33:19).
On whom I have mercy (hon an eleō). Indefinite relative with an and the present active subjunctive of eleaō, late verb only here and Jud 1:23 in N.T. “On whomsoever I have mercy.” The same construction in hon an oikteirō, “on whomsoever I have compassion.”