Definition of Chemical Concentration

In the case of a dissolution chemistry we speak of dilute, concentrated, saturated or supersaturated solutions, terms that refer to the concentration of the solution, specifically to the relationship between the amounts of solute and solvent present in it, however they have a qualitative property. If you want to identify a concentration value, we have different ways of doing it.

Quantification of concentration

The relationships between the amount of solute and solvent or solution can be expressed in various ways, including: molarity,% m / m; % m / v, mole fraction, molality, among others.

Molarity: indicates the number of moles of solute contained in 1 L of solution. If we say that the concentration of an aqueous solution of hydrochloric acid is 0.3 mol / L, it means that 1 L of solution contains 0.3 moles of the solute (hydrochloric acid). In this case, that liter of solution will contain the

volume occupied by 0.3 moles of solute and the remaining volume to reach the liter of solution will be the solvent, Water.

% m / m: indicates the amount of solute in grams, for every 100 grams of solution. Suppose you have an aqueous solution of sucrose at 20% m / m, this implies that you have 20 g of sucrose in 100 g of solution. That is, 100 g of solution are made up of 20 g of sucrose (solute) and 80 g of solvent (water).

% m / v: indicates the amount of solute in grams, per 100 milliliters of solution. Now, let's think about a sucrose solution at 20% m / v, what difference does it make with the previous case? You have 20 g of sucrose in 100 mL of solution. If we knew the density of the solution we could quickly calculate how much mass the 100 represent mL of solution and then know in 100 g of solution how much solute is contained, thus converting% m / v into % m / m.

Molar fraction: it is expressed by a ratio between moles of a solute with respect to total moles, which will take into account, moles of solute plus moles of solvent. For example, if a solution has 3 moles of sucrose and 10 moles of water, the total moles of solution are 13. The mole fraction of sucrose in water will not be other than the quotient between 3 and 13, that is, XS = 0.23. If we want to express it for water, it will be 10/13, XA = 0.77. Note that the sum of mole fractions of a solution must be 1.

Molality: indicates the number of moles of solute per 1 kg of solvent. For example, if you have 30 g of sugar in 200 g of water, the molality will be sought by taking the amount of solvent up to 1 kg or, what is the same, 1000 g. In this way, with a simple proportionality It will be known that for 1000 g of solvent there will be 150 g of sugar, the concentration expressed in molality is m = 150.

How to prepare solutions of different concentrations?

To do this, you must have the right material to do it as accurately as possible. In a chemistry laboratory, there are volumetric flasks, which indicate the exact volume contained, and with granatary balances whose degree of precision is tall. In addition, the measures of security and personal protection objects used based on the risk from work.

In the event that it is desired to prepare a 0.1 mol / L solution of sodium hydroxide, the reagent in the warehouse and a volumetric flask available will be searched. For example, if you have a 500 mL flask, when preparing 500 mL of solution, we must consider that the amount of moles of solute contained in half a liter will be half that expressed in molarity, that is, 0.05 moles of solute. Now, we must weigh this mass in a suitable container, and make sure that the weighed mass only corresponds to the solute and does not contain the mass of the container container.

If you want to weigh 0.05 moles of NaOH, you must know its molar mass, which is approximately 40 g / mol, which means that 0.05 mol is equivalent to 2 g of solute. This mass is carefully transferred to the flask and water is added until the capacity is reached, observing said capacity at eye level. The solution is homogenized and will be ready for later use.

Topics in Chemical Concentration