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 mass
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)
Why is normal saline 0.9%?
- 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.
500ml NS = 4.5g NaCl, i.e. about less than 1 teaspoon of salt in 500ml water.
Teaspoons to Grams Converter
Why is hypertonic saline 5.85%?
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.
References
- Cambridge IGCSE & O Level, Complete Chemistry 4th Edition. Oxford.
- http://web.sahra.arizona.edu
- Saline (medicine). www.wikidoc.org
- Chloride ion. PubChem, the National Library of Medicine
- Irina Chifu et al. Treatment of symptomatic hyponatremia with hypertonic saline: a real-life observational study. European Journal of Endocrinology Volume 184: Issue 5, page647–655
- Teaspoons to Grams Converter, www.thecalculatorsite.com
- Convert Tablespoons of Salt to Grams. Inch Calculator
Dr. Pierre Chan
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