

Pure Water Concentration
Name: Ryan L.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 5/13/2003
Question:
What is the concentration (M) of pure water?
Replies:
I belive it is approximately 55.5 Molar [ moles / Liter ].
Here is one archived question from Newton AAS that verifies my answer:
http://newton.dep.anl.gov/askasci/chem00/chem00005.htm
Regards,
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
Ryan,
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
Regards,
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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