Question

4.0 nanograms per kilogram of water
concentration: ppm

Explanation:

Step1: Convert units

1 kilogram = 10^9 nanograms. 1 ppm (parts - per - million) means 1 part in 10^6 parts.

Step2: Calculate the concentration in ppm

The concentration is 4.0 nanograms per kilogram of water. To convert to ppm, we use the ratio. Since 1 kg = 10^9 ng, the concentration $C$ in ppm is $C=\frac{4.0}{10^9}\times10^6$.
$C = 4.0\times10^{- 3}$ ppm

Answer:

$4.0\times10^{-3}$