醫生爸爸愛足球: From chloride mass to hypertonic saline

2021-09-05

考了兩次化學公開試，廿幾年後，終於知道氯Cl的分子量為35.5的故事。一向以來靠死記，多年來教醫生談及高張生理鹽水的5.85%也提及氯的分子量。

大師兄回來了，我感覺到全都回來了。感謝大女和她的課本。

Chloride is from element Chlorine, which is the 17th element in Periodic Table, with the symbol Cl.

An element contains one kind of atom.
Atoms are the smallest particles which cannot be broken down further by chemical means.
Periodic Table is a way to arrange the elements to show their patterns.

Proton number of Cl is 17. Cl has two natural isotopes.

Isotopes are atoms of same element that have same number of protons, but different numbers of neutrons.
The neutron numbers of 35Cl and 37Cl are 18 and 20 respectively.
35 and 37 are Cl's mass number, which is the total number of protons and neutrons in a nucleus of an atom.

75% of the atoms in chlorine have a mass of 35, and 25% have a mass of 37.

The relative atomic mass of Cl

= 75% x 35 + 25% x 37

= 26.25 + 9.25

= 35.5 (35.45 to be exact)

In medicine, normal saline is a solution of 0.9%.
0.9% is w/v which means weight / volume. i.e. 9g NaCl dissolved in 1 liter water
the molecular mass of Na and Cl are 23 and 35.5 respectively, 23+35.5 = 58.5, the concentration is 9g/L divided by 58.5g/mole = 0.154 mole/L
Since NaCl dissociates into two ions – sodium and chloride – 1 molar NaCl is 2 osmolar. It contains 154 mEq/L x2 = about 300 mOsm/L, which is said to be isotonic with blood in clinical contexts, although it is a technical inaccuracy.

A tablespoon of salt is equal to 3 teaspoons of salt.

500ml NS = 4.5g NaCl, i.e. about less than 1 teaspoon of salt in 500ml water.

5.85%

= 58.5g NaCl dissolved in 1 liter water

= 58.5g / (58.5 g/mole) in 1L

= 1 mole of NaCl in 1 L

= 1 mmol of NaCl in 1ml

i.e. For every ml of 5.85% NaCl aspirated, 1mmol NaCl is expected.

It becomes user friendly in clinical setting.

Dr. Pierre Chan