The volume of CO2 evolved on heating 50 g of CaCO3........................

Explanation

It is the number of parts of one component per million parts of the solution. It can be in terms of mass or volume. Let A and B the two components of the solution then,
`"ppm"=("Mass of A")/("Mass of solution")xx10^(6)or"ppm"=("Volume of A")/("Volume of solution")xx10^(6)`