10.30 mg of O₂ is dissolved into a liter of sea water of density 1.03 g/mL. The concentration of O₂ in ppm is__________.

Solution

Chemistry ma’am · 4 steps

ppm is a mass ratio here, so first turn that litre of sea water into grams using its density.

1. What ppm means. Parts per million by mass is the mass of solute per 10⁶ g of solution. Both masses must be in the same unit.

   ppm = (mass of solute)/(mass of solution) × 10⁶

2. Mass of solution. 1 L is 1000 mL, so the sea water has mass 1000 mL × 1.03 g mL⁻¹ = 1030 g. The dissolved O₂ is 10.30 mg = 10.30 × 10⁻³ g, which adds almost nothing to this mass.
3. Put in numbers. ppm = (10.30 × 10⁻³)/1030 × 10⁶ = 10⁻⁵ × 10⁶ = 10.
4. So. The concentration of O₂ in the sea water is 10 ppm.

The common slip: 10.3. Ignoring the density and taking 1 L as 1000 g gives 10.3 ppm, not 10. Sea water is denser than pure water, so it has a mass of 1030 g.

The idea to keep. For ppm by mass, divide solute mass by solution mass in the same unit. When a volume is given, use the density to get the mass first.

Answer from the NTA final answer key. Solution by PrepWiser.