Question

High concentrations of ammonia (NH3), nitrite ion, and nitrate ion in water can kill fish. Lethal concentrations of these species for rainbow trout are approximately 1.002 mg/L, 0.436 mg/L, and 1362.4 mg/L, respectively. Express these concentrations in molality units, assuming a solution density of 1.00 g/mL.

Answer 1

All of these are done the same way. I'll do NH3.
1.002 mg/L and you want to convert to mols/kg solvent.
You have 1000 mL. The density is 1.00 g/mL so 1,000 mL has a mass of 1,000 grams. That is mass H2O + mass NH3 = 1,000 grams. The mass of NH3 is 1.002 mg or 0.001002 g. So mass H2O = 1,000 - 0.001002 = ? which is essentially 1,000 grams or 1 kg solvent. That will be true for nitrite ion too but probably will not be true for nitrate.
Then mols NH3 = grams/molar mass = 0.001002/17 = ?
Then molality = mols/kg solvent = ?
Post your work if you get stuck.