Exercise 1 solution

Salinity 35 g/kg means 35 g of dissolved salt in each kilogram of seawater.
Chloride = 0.55 x 35 = about 19.3 g.
Evaporating 1 kg of the water to dryness leaves the dissolved salts: about 35 g (with some water of crystallisation and a little loss of gases and volatile material, which is ignored here).

So a kilogram of seawater is only about 3.5 per cent salt, and chloride is a little over half of that.