In past it was normal for chemists to quote concentrations as (weight of solute) / (volume). Hence, the mole is now the most common way in chemistry of showing the quantity of a substance, therefore molar concentrations are almost used everywhere.

What is Molarity ?

 Molarity (M) is defined as the number of moles of solute (the material dissolved) per liter of solution.  Molarity is mostly used to show the concentration of a solution.

Molarity = moles of solute/liters of solution


The units of Molarities are moles per cubic decimeter, written mol dm-3 or simple M. The cubic decimeter is identical to the liter; in many older textbooks you will find that concentrations are written in form of moles per liter or written as mol l-1 e.g.

2.0 mol dm-3 or 2.0 mol l-1 or 2.0 M all stand for the same concentration.

Calculate molarity:

Pure water has its maximum density of 1.000g/mL at 4 degrees Celsius so in 1L there will be 1000g

Molarity = mass per L / molar mass

Number of mole of water=1000g/18g mol

                                   = 55.55 mol / Litter

Molarity of water = n/V

55.55 mol/ 1Litter =55.55M

There’s a real molar concentration for water by itself (55.55 M)

1 L of water at 25 °C , the density of water is 0.9970749 g/mL, so that’s 0.9970749 kg.

At that amount of water, the number of moles is 997.0749 g18.0148 g/mol = 55.55 mols

We can say as a consequence water is 56 molar.  It may depends on temperature because the mass of IL water is depends upon temperature as

At 4 °C                   = 1000gram

At 20 °C                 =998.2 g

At 30 °C                 =995.7 g

At 100 °C                = 958.4g If the formula of molarity is Molarity = mass per L / molar mass then we can easily understand that with change of molar mass molarity will be changed automatically.

Number of moles =100/18 = 55.55 Mol

Molarity             = n / V = 55.55mol/ 1L =55.55 M

Molarity of water at 20 °C

Number of moles =998.2 g/ 18g =55.45 mol

Molarity             = n / V = 55.45mol/ 1L =55.45 M

Molarity of water at 30 °C

Number of moles =995.7 g/ 18g =55.31 mol

Molarity             = n / V = 55.31mol/ 1L =55.31 M

Molarity of water at 100 °C

Number of moles =958.4g/ 18g =53.24 mol

Molarity             = n / V = 53.24 mol/ 1L =53.24 M

So we can say molarity of water can change with change of Temperature.