what is the [a] (mol/l) where [hpo 4 2− ]/[a] (%) is 0.612?
CREATE TABLE "aqueous_solution" (
  "a_mol_l" text,
  "p_h" text,
  "h_3_po_4_a_pct" text,
  "h_2_po_4_a_pct" text,
  "hpo_4_2_a_pct" text,
  "po_4_3_a_pct" text
);
SELECT COUNT("a_mol_l") FROM "aqueous_solution" WHERE "hpo_4_2_a_pct"='0.612';