Molarity of Pure Water

OVERVIEW

Molarity is the number of moles present in a liter of solution. Suppose 10 moles of a solute present in a liter of solution, the solution is 10M in terms of solute concentration. 

Pure water does not have any other substance in it. It is pure water (H2O). In this case, the molarity of pure water would be the number of moles of H2O (molar mass: 18 g/mole) present in 1 liter of water (according to definition of molarity).

Step 1: Calculate the weight of 1 liter (or 1000 ml) of pure water. 

Volume = weight / density

OR

Weight = Volume x density

Known values:
Density of pure water (at room temperature, 25°C) = 0.997 g/ml
Volume: 1000 ml

Put these values in the equation: 

Weight = 1000 x 0.997 = 997 g

Result: Weight of 1 liter (1000 ml) water is 997 g.

Step 2: Calculate how many moles of water in 997 grams of water.

Known values:
Molar mass of water = 18 g/mole

18 g of water = 1 mole
1 g of water = 1 /18 moles
997 g of water = 997 x 1 / 18 = 55.34 moles

Step 3: Calculate the molarity of pure water
Molarity is moles/liter i.e., number of moles present in a liter of water.
55.34 moles of water is present in 997 g or 1 liter of water.
So the molarity of pure water at room temperature is 55.34 M.

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