A250-ml aqueous solution contains 1.56 10–5 g of methanol and has a density of 1.03 g/ml. what is the concentration in ppm?

First, calculate for the mass of the aqueous solution by multiplying the given volume (in mL) by the density (in g/mL). In mathematical equation, that is,

        m = ρV

where m is mass, ρ is density, and V is volume. Substituting the known values,

      m = (1.03 g/mL)(250 mL) = 257.5 g

To get the concentration in ppm, divide the given mass of methanol by the mass of the solution. Note that the parts-per million (ppm) is equal to mass of solute in milligram(mg) divided by the mass of solution in kilogram (kg)

      C (in ppm) = (1.56 x 10^-6 g)(1000 mg/1 g)   / (257.5 g)(1 kg/1000 g)

Simplifying,
     C (in ppm) = (1.56 x 10^-3 mg)/ 0.2575 kg

     C (in ppm) = 0.00606 ppm

0.00606 ppm