How much 5% saline should be given to a 35kg patient to raise the plasma sodium by 5 mmol/l?
Total body water = 35 x 0.6 = 21 litres
21L x 5 mmol/l = 105 mmol
Amount of 5% saline (856 mmol/l) needed = 105 / 856 = 0.123 L
Plus any ongoing renal losses of sodium.