Determining the Mass Percent Composition in an Aqueous Solution | Protocol
Density and concentration both describe the amount of a solute per unit volume of a solvent. The former value measures mass per volume. Concentration in % (w/v) = x (mass solute in g) / (volume solution in mL). Volume = Mass / B. Mercury has a density of g/mL. How many grams of. If there is a relation between the concentration of a substance in a solution and the density of the solution, the concentration can be determined by measuring the .
Percent by Mass - Direct Place a small volume of a solution into a clean and oven-dried beaker or crystallization dish. After accurately determining the precise total mass of the solution, heat the beaker or dish on a hotplate or in an oven to drive off the water. Slow evaporation is the best method, as boiling can result in splattering of the solution. Once the solvent has evaporated, cool the remaining solid solute and determine the mass. Calculate the mass percent as: Percent by Mass - Using a Calibration Curve Make a series of standard solutions by dissolving known quantities of solute into a solvent.
Five standards are recommended and should range from the minimum to the maximum expected percent compositions. Reference tables are available to indicate the maximum solubility for many solids in water over a variety of temperature ranges.
Because mass is additive, this would be the same as adding enough water so the total solution has a mass of g.
Difference Between Concentration and Density
After measuring the mass of a precise volume of each of the standard solutions, calculate the density as: Data Analysis Plot the density values against the percentage by mass of the standard solutions and determine the slope of the line.
This slope corresponds to the density vs. Calculate the density of the unknown sample solution by obtaining the mass of the specific volume of solutions. Now that the slope of the line is known, determine the percent by mass of solute by solving for "x" using the measured density and slope from the calibration curve.
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. 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.
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. 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.
Therefore, as a fixed amount of volume is extracted from the standard solution, a fraction of the moles of the standard is being transferred into the new solution. When the solution is made into a mL solution by adding distilled water, the number of moles in the flask is retained but a decrease in concentration is observed due to the increase in volume. Hence, since the number of moles remains the same in both solutions but the volume increases, concentration decreases.
This relationship can be shown by the following equation: The relationship between concentration and conductivity can then be found by measuring the conductivity of these solutions. A conductivity meter is used for the accurate measurement of conductivity.
Approximately 5mL of the solution would be poured out into a smaller cup and its conductivity is measured. Since the 5mL solution is taken to represent the conductivity of the entire solution, extra care has to be taken to ensure that the solution was swirled thoroughly.
Also, the metal rods on the conductivity meter that takes the measurements have to be cleaned and dried properly.
Contamination of the rod will lead to errors in the reading of conductivity while excess distilled water left on the rods after drying may dilute the solution and would defeat the purpose of the experiment. With the results obtained, a conductivity-concentration curve can be constructed and linear interpolation can be carried out to find the conductivity of the unknown solution. This acts as the standard solution for the salt. Repeat this two more times until there is 30mL of standard solution in the flask.
Mass concentration (chemistry)
This is the first dilution needed of the standard salt solution. Now there should be three salt solutions of different concentrations. It is important to label each of the volumetric flasks. Determining Conductivity of Solutions 1 The metal rods on the conductivity meter were washed with distilled water 2 The rods were then wiped dry with Kimwipes.
Assuming the conductivity of the extracted solution to be that of the standard solution. Weighing the Solutions 1 Label, clean with RO water, dry and weigh a pycnometer 2 Fill pycnometer with water, dry with a Kimwipe the outside surface of pycnometer and weigh it.
Make sure to dry every spot where there may be any fluid bottom, treads, etc. The presence of fluids anywhere other than inside the pycnometer will influence the weight and produce error. Furthermore, it is key that the droplet at the tip of the pycnometer remains somewhat constant throughout each measurement to ensure a constant volume.
Difference Between Concentration and Density
Lastly, do not put the Kimwipe directly into the hole of the pycnometer because it will absorb fluid found inside the pycnometer. Reference source not found. The table suggests that a greater concentration of salt leads to greater density of Table 4 — Densities of Compounds at solution. This falls within reason because as is 22OC shown in Table 4, the densities Sigma-Aldrich of Compound Density all the salts are well above the density of water.
Another trend observed from Error! Reference source not Na2SO4 2. Once again this can also be described by the density values given in Table 4. Since Na2SO4 has the greatest density out of the three salts, a specific concentration in solution will result in a greater overall density of solution than any of the other salts at the specific concentration. This also explains why KCl has the lowest density at a specific concentration for all three solutions, because it has the lowest density of the salts.
The two trends observed, density being proportional to concentration and the different densities given the salts, are based on the fact that the greater the density of a salt is, the more of an effect one mole of that salt will have on changing the density of the solution.
This is particularly true because the solvent, water, has a much lower density than any of the salts used in this particular experiment. The table Graph 2: Apart from minor deviations from the regressions, the relationships are all linear, which can be compared to true values, which on small scalesgive a linear relationship as well.
This observation is explained by analyzing each of the salts used. Our tests indicate that Na2SO4 also has the highest conductivity in solution, and therefore one can conclude that conductivity is proportional to the number of ions that dissociate in solution from one mole of salt.
But this does not describe the difference between the conductivities of the NaCl and KCl solutions. During the experiment, an unknown salt solution was given, with the purpose of finding out its approximate concentration and type of salt, from the three salts used Na2SO4, KCl, NaCl.
Using the same testing techniques used throughout this lab the conductivity and density were determined. The necessary concentrations would be: To determine which type of salt is present in the unknown solution, use linear interpolation of the different concentrations against the known conductivity 7.
Graph 4 Given the above Graph 1 graph, it is easily discernable that the salt used in the unknown solution is Na2SO4. Given that it is the only salt concentration that intersects with the experimental conductivity of 7. Therefore given all the information that has been extracted from the graphs it can be determined that that unknown solution was a Na 2SO4 solution, with an approximate concentration of 0.
Na2SO4 at a concentration of 0. According to the graphs, the theories stated in the introduction are correct: