Rainwater collected in akron contains pb, cu and zn at 8.5, 5.0 and 7.5 ug/l. determine these concentrations in units of mol/l (or nmol/l). b) this same rainwater contains 0.8 mg/l ca and 1.2 mg/l mg. determine the hardness of this water in units of mg/l as caco3. hardness is the sum of the concentrations of multivalent cations in the water, which in this case include ca (as ca2+) and mg (as mg2+). hardness is expressed in "equivalent" units or "caco3" units.