what is the mass of KCl in a 0.000198ppm solution?
= 0.000198 * 1kg / 10^6
= 1.98 * 10^-10 g = 0.98ng (nanograms)
I did it, correct, answer?
Please check it for me.

Calculate the number of moles of KCl in a 0.000198 ppm solution.
moles = grams / molar mas = 1.98 * 10^-10 / 74.55 = 2.7 * 10-12 moles
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1. It is in grams per litre (or kilograms per metre cube [you forgot to convert kilograms into grams]), so it gives 19.8 x 10^-6 g/L (or g/dm^3) or 19.8 \mu g/dm^3

2. Then, it gives 2.65 x 10^-9 mol/dm^3.

'ppm' is a measure of concentration...