1.03g/mL=1.03kg/L.
1.5E21 L has a mass of 1.5×1.03E21=1.545E21kg.
3.1%=0.031 of this is sodium chloride=0.047895E21=4.7895E19 kg.
(E19 means ×10¹⁹)
A metric tonne is 1,000kg so the mass is 4.7895E16 tonnes.
1 ton=1.01696 tonne. The mass in imperial measure is 4.71 ton approx.