Question

Water treatment plans must reduce phosphate levels to 3.2 micromoles PO4^3-/L before the water is considered drinkable. If water from Lake A is determined to have a phosphate concentration of 0.419 mg PO4^3-/L, does it meet this allowable standard for drinking water?

Answer 1

1 mol PO4^3- = about 95g but you can do that more accurately.
0.419 mg = 0.000419 g =
0.000419/95 = about 4.4E-6 mols.
That is 4.4 micromols.