# Calculating salinity from sodium and chloride in water [closed]

let me start by saying that I am definitely no chemist, so this question might (or might not) seem trivial to all you professionals out there.

I have recently found a brand of carbonated water here in Norway that I really like. It is quite salty, compared to your regular stuff, and I want to know the exact salinity (in parts per thousand) of this water. The minerals contained are as follows (in milligrams per liter):

• sodium 455
• magnesium 31
• calcium 28
• potassium 16
• chloride 667
• bicarbonate 300
• sulphate 11
• silicon 6

As the chloride forms bonds not only to the sodium, I would assume it isn't as easy as just adding the chloride and the sodium, and I do not know how much NaCl this would equate to. Can you help me?

You can't really balance cations and anions with the information given to get precise compound concentrations. However some "back of the envelope" calculations can be done.

To compare the various ions you need to reduce the concentrations to a common unit. A convenient one would be millimoles per liter. To get millimoles per liter you need to divide the concentration of milligrams per liter by the molar mass of each of the different kinds of ions. (A mole is just a count of atoms like a dozen. However the mass of an atom is vanishingly small so you need a lot of them to get a convenient mass. So one mole actually has about $$\mathrm 6.022*10^{23}$$ atoms.)

sodium 455/22.99 = 19.8 millimoles/liter

chloride 667/35.4 = 18.8 millimoles per liter

So the sodium and chloride are roughly the same about 19 millimoles per liter. This is well below what is considered safe levels for drinking water in the USA.

Edit

Since we're doing back of the envelope calculations, the milligrams of sodium and chloride correspond to roughly the same number of atoms in the water. So the mass of sodium and chloride can be added to get:

$$\mathrm{0.455 + 0.667 \approx 1.1}$$ grams NaCl per liter.

Since a liter of water has a mass of about 1 kilogram, that's 1.1 ppt of NaCl.

• Hmm, but is not salinity defined a the mass of dissolved salts per the unit mass (approx volume) of water? Like 500 mg/kg or 500 mg/L? In such a case, he could just summarize masses of provided ions to get the approx. number. Jul 11 at 7:12
• Thank you. As I understand it, being a non-chemist, I still don't know how I go from mmol/L of NaCl to mg/L, would that simply be reversing the calculation? Thus, I end up with approx. 500 mg/L, or a salinity of about .5 ppt? Jul 11 at 7:52
• Scrap that, recalculations considering the molar weight of water gives me a salinity of 0.34 ppt of NaCl. Sounds reasonable? Jul 11 at 7:59
• @Poutnik - I didn't know what the OP really wanted. It seems now that he wanted ppt of NaCl which I added to the answer.
– MaxW
Jul 11 at 8:18
• @MaxW Thank you! This would actually mean this water would be considered brackish, as it is >.5 ppt. Interesting. Jul 11 at 8:37
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