Answer is: concentration of silver chloride is 1.5 ppm.
m(AgCl) = 1.2·10⁻³ g = 0.0012 g.
m(AgCl) = 0.0012 · 1000 mg/g.
m(AgCl) = 1.2 mg.
m(solution) = 800 g ÷ 1000 g/kg.
m(solution) = 0.8 kg.
concentration = 1.2 mg ÷ 0.8 kg = 1.5 mg/kg = 1.5 ppm.
Parts-per-million (10⁻⁶) is present at one-millionth of a gram per gram of sample solution, for example mg/kg.