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Molar Mass of a Solid Essay


The purpose of this experiment is to determine the molar mass of an unknown solid. The freezing point of a solvent (Cyclohexane) was determined, and compare to the freezing point of Cyclohexane with the addition of two different concentration of unknown solid. The pure substance of Cyclohexane has defined physical properties. However, when a solute is added these properties are determined by the amount of solute added, also known as colligative properties. The temperature of the freezing point was lowered when the solid was added and it decreased as the concentration of the solid increased. The molality of the solute was determined, which was then used to calculate the molar mass of the solid for both trials. An average of the molar mass of the unknown solid was calculated to be 286.87+1.796g/mol with a %RSD of 0.626%. The purpose of the experiment was to obtain the molar mass of an unknown solid.

The molar mass is determined by dissolving the unknown compound into a pure liquid with defined physical properties, in this case Cyclohexane. When dissolving a solute, it forms a homogeneous solution, in which the solvent of the solution physical properties are not longer defined, but depends on the amount of solute added. Whenever a substance is dissolved in a solvent, the vapor pressure decreases. When the vapor pressure decreases, it also lowers the boiling and freezing point of the solvent and increases the osmotic pressure. These four properties are known as the colligative properties, the magnitude of the changes depends on the amount of solute dissolve; the changes are directly proportional to each other. The higher the solute concentration, the higher the changes of vapor pressure, therefore, a lower freezing point. It is hypothesized that the freezing point temperature will decreased, if there is a higher concentration of the solid added to the soution. Vapor pressure is the pressure exerted by a vapor when the vapor is in a state of dynamic equilibrium with its liquid. When a nonvolatile solute is dissolves in the solvent, solute molecules occupy a part of the surface area.

This allows molecules into the vapor state, lowing the vapor pressure of the solvent. When the vapor pressure lowers, the boiling point elevates, due to the presence of the solute. Solute added to the solvent affects the freezing point, the energy of the solvent lowers to increase the magnitude of the intermolecular forces for the molecules to get together to form a solid. The colligative properties are useful for determining molar masses of unknown nonvolatile solute. At low concentrations of a nonvolatile solute, the freezing–point of depression of a solvent can be determine by Tf = Kf (solvent) -Tf (solution) = Kf m. Changes in freezing point Tf are directly proportional to the molality, m, of the solute in solution (molality, m = mol solute/ kg solvent). The freezing-point constant, Kf for Cyclohexane is 20˚C • k/mol, which is used to obtain the amount of moles of solute in the solution, which is inversely proportional to the change of freezing point Tf.

The molal mass obtained is essential to obtain the molar mass of the unknown solid (solute). The molal mass shows the unknown amount of moles of solute are need it per kilogram of solvent. Acquiring the number of moles of solute and analyzing it with the grams of solute measured in the laboratory obtaining the molar mass of the unknown solute.i The cooling behavior of a solution different from a pure liquid solution when a solute it’s added. Te temperature at which the solution begins to freeze is lower than the pure solvent. The freezing point also lowers when there is a higher concentration of solute added to the solution. The molar mass of unknown solid can be determined by any of the colligative properties, the freezing point is not an exception and by knowing the mass of solute and temperature of the freezing point, the molar mass can be obtained.


The freezing point depression is a colligative property of solutions, which is based only on the amount of solute added and not the defined chemical and physical properties of the substance. The freezing point of Cyclohexane observed was 4.97˚C, from the cooling curve (Figure 1). Freezing point values of Trial 1 and Trial 2 were collected, to compared to the freezing point of Cyclohexane and determine he depression of the solute. The unknown solid yielded a freezing depression of 3.64˚C for trial 1, and 2.84˚C for trial 2 (Figure 2 & 3). These values were compared to the original mass of unknown solute added to the Cyclohexane and the approximate molar mass of the unknown solid.

The molar mass that was calculated for trial 1 was 288.14-g/mol, while trial 2 was 285.60-g/mol (Table 3). The average of the molar mass was calculated, and the molar mass of the unknown solid observed was approximately 286.87-g/mol. When calculating the standard deviation was obtain only +1.796g/mol when comparing both trials to the mean. The values are apparently close up together, making them highly precise, however, is unknown if the values are accurate, the unknown solid actual molar mass still unknown. The relative standard deviation percent, obtained was 0.626% (Table 3); this measurement expresses precision of the data, the smaller RSD the greater the precision for the average value of the data.

The %RSD, is extremely low, making it the results a tremendously precise data of the mean. However, there was a possible error that could have affected the results. The error occurred during recording the cooling temperature of trial 1, the ice melted and the test tube was removed to add more ice into the beaker. Also, while recording the temperature of the Cyclohexane upon addition of unknown solid, the sample was not stir and therefore the value could have probably been invalid. Constant stirring of solution would have assured an even mix of the solute particles and a more accurate freezing point. These small errors could have affected both trials creating a higher observed molar mass value for both trials.


The experiment allowed for the determination of freezing point of Cyclohexane based on a logger pro software with calibrated thermometers to be approximately 4.97˚C (Table 1). Freezing point depression was observed when two trials of different concentration of the unknown solid were added to the Cyclohexane. As the concentration of the unknown solid increased, the freezing point of the solution was also lowered (Table 2). As seen on Figure 4, when placing all three graphs it is visible, when solute is added a lower freezing point is observed. Using experimental measurements, the molar mass of the unknown solid was calculated to be 286.87+1.796g/mol. Based on the relative standard deviation percent (%RSD) of 0.626%, shows a highly precise measurements of unknown solid molar mass when compared to the mean values.

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