Mole and Molarity
The most common way of expressing concentration in a chemistry laboratory is to use molarity. As discussed previously in Section 3.7, a solution’s molarity is given by the number of moles of solute per liter of solution (mol/L, abbreviated M).
Molarity (M) = Moles of soluteThe advantages of using molarity are twofold: (1) Stoichiometry calculations are simplified because numbers of moles are used rather than mass, and (2) amounts of solution (and therefore of solute) are measured by volume rather than by mass. As a result, titrations are particularly easy.
The disadvantages of using molarity are also twofold:
(1) The exact concentration depends on the temperature because the volume of a solution expands or contracts as the temperature changes
(2) the exact amount of solvent in a given volume can’t be determined unless the density of the solution is known. (Remember that solutions of a given molarity are prepared by dissolving a solute in a small amount of solvent and then diluting with solvent to the desired volume. The solution is not made by dissolving the solute in the desired volume of solvent.)
(1) The exact concentration depends on the temperature because the volume of a solution expands or contracts as the temperature changes
(2) the exact amount of solvent in a given volume can’t be determined unless the density of the solution is known. (Remember that solutions of a given molarity are prepared by dissolving a solute in a small amount of solvent and then diluting with solvent to the desired volume. The solution is not made by dissolving the solute in the desired volume of solvent.)
Worked example
A total of 58,5 g of NaCl dissolved in water so that the volume of solution to 500 mL. What molarity NaCl solution ? (molar mass = 58,5 g/mol)
Answer :
Change the unit of mass (g) into the mole, then count molarity of solutionSo, Molarity of NaCl solution = 2 M
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