An equivalent is described as an amount of substance that in an acid base reaction reacts with or supplies 1 mole of H+ ions or in a redox reaction supplies or reacts with 1 mole of electrons.
For Na+, as 1 mole of Na+ reacts with 1 mole of electrons in a redox reaction, one equivalent of Na+ is one mole of Na+.
The patient receives an IV fluid that contains 154 mEq of Na+ per liter of the solution and is given 500 mL of the solution.
The mEq of Na+ that the patient receives is 154*0.5 = 77 mEq.
77 mEq of Na+ is 77*10^-3 mole. The patient is given 77*10^-3 mole of Na+.