A person suffering from hyponatremia has a sodium ion concentration in the blood of 0.118 M and a total blood?

Question

volume of 4.6 L. What mass of sodium chloride would need to be added to the blood to bring the sodium ion concentration up to 0,138 M, assuming no change in blood volume?

Answer 1

Simonizer's calculations are correct if you are only considering the blood volume, but in reality it doesn't work like this and you'd have to add a lot more sodium. This is because sodium is not just present in the blood, it is distributed throughout the total body water (or at least it behaves as if it is). Whatever sodium you give will not stay in the blood, it will spread out throughout the water in the body. You can estimate total body water. It's usually about 60% of a man's weight or 50% of a woman's weight. So in a 70kg man that would be 42kg, which is 42L.

0.118M in 42L is 4.956 moles (the amount the patient has now)

0.138M in 42L is 5.796 moles (the amount you want the patient to have)

So the man has a sodium deficit of 0.84 moles.

If you give 0.84 moles of sodium chloride, this will be 49g of NaCl.

(from 0.84 x 58.4, which is the molecular mass of NaCl)

49g of NaCl is approximately one litre of 5% saline.  

Answer 2

Molar mass of NaCl is 58.4 gms/mol

0.118 moles/L x 4.6 L =  0.5428 moles of NaCl = 31.7 grams of NaCl

For 0.138 M, you need 0.138mole/L x 4.6 L = 0.6348 moles

0.6348 x 58.4 gm/mol = 37.1 grms of NaCl

37.1 - 31.7 = 5.4 grams to be added