Molarity
The molarity of a solution is the concentration of the solution expressed in terms of the number of moles of solute in one liter of solution.
The molarity of a solution is given the symbol M. It can be calculated by dividing the number of moles of solute by the volume of the solution in liters.
It is important to note the distinction between molarity and molality. Molarity is the concentration in terms of the volume of solution and molality is the concentration expressed in terms of the mass of the solvent.
The molarity of a solution is the most commonly used way of describing the concentration. An example of this would be describing a solution as a 1 molar solution, or saying it had a strength of 1 M. This would mean that for every liter of solution there is one mole of solute. It can be seen from this that the 1 M solution has one mole per liter, so the symbol M stands for moles per liter. If we are aware of the molarity of a solution we can calculate the number of moles of solute in a given volume of solution. It can then be seen that the molarity of a solution is a conversion factor between the volume of the solution and the number of moles of solute.
To illustrate the above, 4 L of a solvent and 2 moles of a solute would be able to produce a 0.5 M solution (2 divided by 4). The number of moles present in any amount of the solution can be calculated by multiplying the molarity by the volume. For example, in 3 L of the 0.5 M solution there are 1.5 moles (0.5 M x 3 L).
It should be noted that the volume of a solution is dependent upon the temperature. At higher temperatures the solution will expand and at lower temperatures the solution will contract. As a result, the molarity of a solution is a temperature- dependent quantity. This is different from molality which is a temperature-independent quantity. For maximum accuracy, the temperature the molarity was measured at should be stated.
If the solvent used is water the molarity and the molality are essentially the same. This is because 1 kg of solvent is nearly equal to 1 kg of solution and the nature of water's density means that 1 kg of a dilute solution will be very close to 1 L in volume.
For dilute solutions the volume of the solvent is essentially the same as the volume of the solution and most calculations are made with this assumption.
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