Unit ppm stands for parts per million. in terms of mass, ppm is equivalent to mg/kg. since 1 kg is 10⁻⁶ mg, 1 kg is equivalent to million mg. therefore mg/kg is also ppm. there are 1.3 g of Ca ions in 3100 kg if 3100 kg contains - 1.3 g of Ca then 1 kg contains - 1.3 g / 3100 kg then Ca ions - 0.42 x 10⁻³ g/kg Ca ion concentration - 0.42 mg/kg therefore Ca ion concentration is 0.42 ppm
