Asample of seawater contains 1.3 g of calcium ions in 3,100 kg of solution. what is the calcium ion concentration of this solution in units of ppm?

A unit of ppm has an acronym of parts per million. The equivalent units of ppm is therefore mg/L and mg/kg. So the ppm is:

mass Ca+ = 1.3 g = 1300 mg

ppm = 1300 mg / 3100 kg

ppm = 0.42 ppm