Molarity is a way of expressing concentration that is equal to the ratio of the moles of the solute to the liters of solution. The amount of substance in moles can be calculated through,
moles = (molarity) x (volume in liters)
Substituting the values,
moles = (0.135 M) x (5.0 L) = 0.675 moles
Multiply this value with the molar mass of sodium ion,
mass = (0.675 moles) x (23 g/ mole) = 15.525 grams
Therefore, the mass of sodium ion in the blood is 15.525 grams.