Concentrations of solutions may also frequently be given in terms of the weight of solute in a given volume of solvent or solution. The Columbia Electronic Encyclopedia, 6th ed. All rights reserved. See more Encyclopedia articles on: Chemistry: General. Enter your search terms:. The concentration of a solute is very important in studying chemical reactions because it determines how often molecules collide in solution and thus indirectly determines the rates of reactions and the conditions at equilibrium see chemical equilibrium.
It also uses conversion factors to convert between grams and moles and between milliliters and liters. This video looks at how to use molarity as a conversion factor. If you know the molarity, you can solve for either the number of moles or the volume of a solution. Also, molarity is a ratio that describes the moles of solute per liter of solution. Stoichiometry can be used to calculate the quantitative relationships between species in aqueous solution.
Calculate concentrations of solutions in molarity, molality, mole fraction and percent by mass and volume. Recall that a solution consists of two components: solute the dissolved material and solvent the liquid in which the solute is dissolved. The amount of solute in a given amount of solution or solvent is known as the concentration. The two most common ways of expressing concentration are molarity and molality.
The molar concentration M of a solution is defined as the number of moles of solute n per liter of solution i. For example, the number of moles of NaCl in 0.
The molal concentration m of a solution is defined as the number of moles of solute n per kilogram of solvent i. For example, the number of moles of NaCl dissolved in 0. We can perform stoichiometric calculations for aqueous phase reactions just as we can for reactions in solid, liquid, or gas phases. Almost always, we will use the concentrations of the solutions as conversion factors in our calculations.
Crystals of silver chloride AgCl : Stoichiometry deals with the relative quantities of reactants and products in chemical reactions. It can be used to find the quantities of the products from given reactants in a balanced chemical reaction, as well as percent yield. The next step, as in any calculation involving stoichiometry, is to determine our limiting reactant. We can do this by converting both of our reactants into moles:. Because there are fewer moles of NaCl present in solution, NaCl is our limiting reactant.
We can now solve for the mass of AgCl formed:. Privacy Policy. Skip to main content. Aqueous Reactions. Search for:. Learning Objectives Calculate the concentration of a diluted solution. For many applications this may not be a problem, but for precise work these errors can become important.
In contrast, mole fraction, molality, and mass percentage depend on only the masses of the solute and solvent, which are independent of temperature. Mole fraction is not very useful for experiments that involve quantitative reactions, but it is convenient for calculating the partial pressure of gases in mixtures, as discussed previously.
Mole fractions are also useful for calculating the vapor pressures of certain types of solutions. Molality is particularly useful for determining how properties such as the freezing or boiling point of a solution vary with solute concentration.
Because mass percentage and parts per million or billion are simply different ways of expressing the ratio of the mass of a solute to the mass of the solution, they enable us to express the concentration of a substance even when the molecular mass of the substance is unknown.
Units of ppb or ppm are also used to express very low concentrations, such as those of residual impurities in foods or of pollutants in environmental studies. Vodka is essentially a solution of ethanol in water.
The density of pure ethanol is 0. If we assume that the volume of the solution is the sum of the volumes of the components which is not strictly correct , calculate the following for the ethanol in proof vodka. Asked for : mass percentage, mole fraction, molarity, and molality.
We can then calculate the number of moles of ethanol and the concentration of ethanol in any of the required units. A Because we are given a percentage by volume, we assume that we have The volume of ethanol will thus be The mass of ethanol is obtained from its density:. If we assume the density of water is 1. We now have all the information we need to calculate the concentration of ethanol in the solution.
B The mass percentage of ethanol is the ratio of the mass of ethanol to the total mass of the solution, expressed as a percentage:. C The mole fraction of ethanol is the ratio of the number of moles of ethanol to the total number of moles of substances in the solution. D The molarity of the solution is the number of moles of ethanol per liter of solution. We already know the number of moles of ethanol per The molality of the solution is the number of moles of ethanol per kilogram of solvent.
Because we know the number of moles of ethanol in A solution is prepared by mixing The densities of toluene and benzene are 0. Assume that the volume of the solution is the sum of the volumes of the components.
Calculate the following for toluene. Different units are used to express the concentrations of a solution depending on the application. The concentration of a solution is the quantity of solute in a given quantity of solution. It can be expressed in several ways: molarity moles of solute per liter of solution ; mole fraction, the ratio of the number of moles of solute to the total number of moles of substances present; mass percentage, the ratio of the mass of the solute to the mass of the solution times ; parts per thousand ppt , grams of solute per kilogram of solution; parts per million ppm , milligrams of solute per kilogram of solution; parts per billion ppb , micrograms of solute per kilogram of solution; and molality m , the number of moles of solute per kilogram of solvent.
Learning Objectives To describe the concentration of a solution in the way that is most appropriate for a particular problem or application. To be familiar with the different units used to express the concentrations of a solution.
What is its molarity? What is its mole fraction?
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