Determining the Mass Percent Composition in an Aqueous Solution

Determining and understanding the composition of a solution is fundamental in a wide range of chemistry techniques. A solution is a homogeneous mixture, composed of a substance dissolved in a liquid. The dissolved substance is called the solute, and the bulk fluid is called the solvent. Solutions can be described in several ways — first, by the type of solvent used to dissolve the solute. Aqueous solutions utilize water as the solvent. Organic solvents, such as chloroform, acetonitrile, or acetone, are used to make organic solutions, depending on the properties of the solute.

Solutions are also described by the amount of solute dissolved in a certain volume of the solvent, or the concentration. For more information, see this collection's video on "Solutions and Concentrations".

One way solution concentration can be calculated is based on the mass of each component of the solution, called mass percent. This video will introduce the mass percent of solutions, and demonstrate how to determine the value in the laboratory.

The amount of solute in the solvent can be expressed in a number of ways. First, mass percent is calculated as the mass of the solute divided by the total mass of the solution. Mass percent is written as percent weight/weight, as it is sometimes referred to as weight percent. However, the term mass percent is generally regarded as correct.

The amount of solute in a solution can also be expressed in terms of mass percent by volume, commonly used in biology, and is calculated as the total mass of the solute divided by the volume of the solution. This unit is not a true percent, as it is not a dimensionless quantity. Similarly, volume percent is also used, where the volume of the solute is divided by the total volume of the solution.

Mass percent differs from volume percent, as only mass is an additive quantity. The mass of the solution is the sum of the components. Volume, on the other hand, isn't always additive, as molecules of the solute will fit into open spaces between molecules of solvent, resulting in less volume than expected.

Several simple methods can be used to determine the mass percent of a solution in the laboratory. First, the direct method requires recording the total mass of solution, and then evaporating off the solvent. The solid left behind is the solute, which is then weighed and used to calculate mass percent.

Density can also be used to determine the mass percent of an unknown concentration. First, a series of standard solutions are prepared, with a range of solute amounts. Select standards that range in concentration from no solute, to the maximum solubility of the solute. The density is then calculated for each standard, and compared to that of the sample. Now that the basics of mass percent calculations have been outlined, lets take a look at calculating mass percent using the direct method and the density method in the laboratory.

First, weigh a clean and oven-dried beaker or crystallization dish, and record the mass.

Place a small volume of the solution into the dish, and accurately weigh the solution and dish. Subtract the weight of the dish, and record the mass of the solution.

Next, place the dish on a hotplate or in an oven to drive off the water. Slow evaporation is ideal, as boiling can result in splattering of the solution, and loss of solute. Once the solvent has evaporated, cool the remaining solid, which is the solute. Accurately determine the mass. Calculate the mass percent by dividing the mass of the solute after evaporation by the mass of the initial solution.

First, prepare five standards, with percent compositions ranging from the minimum to the maximum of the expected unknown percent composition. Do not exceed the solubility of the system. Calculate the amount of solute and solvent needed to prepare the percent compositions.

Weigh the solute, and then dissolve it in the specific weight of solvent.

Next, measure the mass of a precise volume of each standard solution. Then calculate density for each solution. Do the same for the unknown solution. See this collection's video on density for more information.

Plot the density values of the standard solutions against the mass percentages. In this example, a set of sodium chloride standards was prepared with mass percent compositions ranging from 5 – 25% of solute in solution. After plotting the data, apply a linear trend-line, and determine the equation, where y is density and x is mass percent composition. Use the density of the unknown solution, in this case 1.053 g/mL, to determine its mass percent. Solving for x, it was found to be 7.4%.

Now that you've seen the basics of determining the mass percent of aqueous solutions, lets take a look at some instances where the concentration of a solution must be known.

Buffer solutions are commonly used in many laboratory experiments, especially in biological applications. Buffer solutions contain a specific concentration of dissolved weak acids or base, and their conjugates, enabling it to maintain and resist changes in pH. Often, buffers are prepared using a mixture of many salts, including those with phosphate groups that maintain a constant pH in the range of 7.2 – 7.6.

Mass percent is also used in order to separate complex mixtures, as with this example of a density gradient. For this, solutions with a range of mass percent are prepared, and then layered sequentially from high mass percent to low mass percent. This creates a density gradient in the tube. The sample is then loaded onto the top of the gradient, and centrifuged. The components of the mixture are then separated based on how far they travel down the tube.

Understanding mass percent is not only critical to chemistry applications, but also to environmental studies. Soil moisture content is important to understand in order to identify environmental conditions that may influence ecosystems. Soil moisture content is a mass percent that describes the amount of water held within the soil. Moisture content is easily found by weighing the moist soil, desiccating the soil it in an oven, and reweighing.

You've just watched JoVE's introduction to determining the mass percent of an aqueous solution. You should now understand the importance of mass percent, its use in the laboratory, and how to determine it for an aqueous solution.

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