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.
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
Theres 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 thats 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.