Example: prepare a 100 mL solution of 1 mg/L of Na+ with NaCl salt
It can be seen here that in 1 mg of NaCl, there is in fact only 0.39 mg of Na+. This ratio must therefore be taken into account when calculating the mass of salt to be weighed in order to obtain a concentration of 1mg/L of Na+.
Thus in order to obtain a 100 mL solution of 1mg/L of Na+, we will need to weigh 0.254 mg of salt. In reality, weighing 0.254 mg of salt is practically impossible because the most precise scales have a precision of 0.1 mg. We will need to prepare a more concentrated solution and then dilute it in order to obtain the desired concentration level of Na+.
Example: prepare a 100 mL solution of 1 mol/L Na+ with Na2SO4 salt
Thus, the mass of salt to be weighed in order to prepare a 1 mol/L solution of Na+ is :