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Name: Ryan L.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 5/13/2003

What is the concentration (M) of pure water?

I belive it is approximately 55.5 Molar [ moles / Liter ].

Here is one archived question from Newton AAS that verifies my answer:

Darin Wagner

I assume you mean the molar concentration (moles/liter). Also assume for simplicity that the molecular weight of water is 18 gm/mol and the density is 1000gm/liter. You could use more accurate data where the effect of temperature changes the density a bit, and the molecular weight of water is not exactly 18 gm/mol. But neither of these small discrepancies have an appreciable effect on the computation:

[1000gm H2O / 1 liter] * [1 mol H2O/ 18 gm] = [1000/18] mol H2O/liter = 55.56 mol H2O/ liter.

Vince Calder


As you know, a litre of water at 4 C has a mass of very near 1000 grams. A mole of water is represented by 18 grams of the liquid. Therefore the concentration of pure water in moles per litre is:

(1000g H20 per L) / (18 g H2O per mole) = 55.55 M/L

ProfHoff 666

Molarity is moles/liter. Water is about 18 grams per mole, and 1000 grams per liter. so divide it out, 1000/18 = 55 moles/liter or so. You can do this more accurately if you get exact numbers off the periodic table etc.

Steve Ross

It depends on the temperature. At 25 degrees C and 1 atmosphere pressure, one cubic meter of water has a mass of 997.0480 kg. To convert this to molarity we need to know the molar mas of water and the volume of the liter. The molar mass of water is 18.015 g/mole, and a cubic meter is 1000 liters. So let us do the math:
(997.0480 kg/m^3)(m^3/1000 L)(1000 g/kg)(mole/18.015 g)
= (997.0480 g/L)(mole/18.015 g)
= (997.0480 g/L)(mole/18.015 g)
= 55.345 mole/L

At higher temperatures and lower pressures it will be less; at lower temperatures (down to 4 degrees C: water expands below that temperature) and higher pressures it will be more.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois

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