Sea water, which can be considered as a 6 molar (6 M) solution of NaCl, has a density of 2 g mL-1. The concentration of dissolved oxygen (O2) in sea water is 5.8 ppm. Then the concentration of dissolved oxygen (O2) in sea water, in terms of molality, is x 10-4 m. x = . (Nearest integer) Given: Molar mass of NaCl is 58.5 g mol-1 Molar mass of O2 is 32 g mol-1
Answer
2
Solution
Take 1 L (1000 mL) of sea water.
Mass of solution = 1000 2 = 2000 g
Mass of NaCl = 6 58.5 = 351 g
Mass of water (solvent) = 2000 - 351 = 1649 g = 1.649 kg
5.8 ppm means 5.8 g of O2 in 106 g of solution.
Mass of O2 in 2000 g solution = 5.8 2000106 = 0.0116 g
Moles of O2 = 0.011632 = 3.625 10-4 mol
Molality = 3.625 10-41.649
= 2.2 10-4 m
x 2
Answer: 2