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CHM120H5 Lecture Notes - Freezing-Point Depression, Molality

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6 Dec 2013

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30 january, 2013: the temperature gradually drops as the solution is freezing because the concentration of the solution increases as the pure solvent in the solution freezes. This is because, in this experiment, we are interested in finding the change in temperature and not static values for temperature. As the temperature changes, the moles of solute to the volume of the solvent ratio changes which will eventually affect the molarity value despite having the same amount of solute in the solution. However, molality is measured in terms of mass of solute to the mass of solvent, which in this case is constant regardless of the temperature. Thus, molality is the unit used in measurements involving changes in temperature: using cyclohexane as a solvent is ideal for this experiment because it has a higher freezing point and freezing point depression constant.

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DETERMINATION oF MOLAR EXPERIMENT 23 MASS FROM FREEZING POINT DEPRESSION EQUIPMENT AND CHEMICALS: Equipment 600 mL beaker Chemicals Cyclohexane (C Stir plate 100 ml. graduated cylinder Clamp Nonvolatile Unknowns Freezing point apparatus Spin vane 0.1 C Thermometer INTRODUCTION: When a solute is introduced into a pure solvent the newly formed solution will have certain physical properties that are from the solvent. As a of the presence of the the pressure of the solution will be lower than that of the pure solvent. As the vapor pressure is lowered there is a resulting increase in the amount of energy required to reach the boiling point of the solution. Conversely, the solution must cool to a lower temperature before a state change from liquid to solid occurs. These changes in physical properties vapor pressure lowering, boiling point elevation and freezing point depression are collectively referred to as colligative properties. These colligative properties, which also include osmotic pressure, do not depend on the nature of the nonvolatile solute in the solution but rather on the concentration of solute present. Because the properties are independent of the nature of the solute, they may be used to determine other properties of the solute such as molecular weight. The effect of a nonvolatile, nonelectrolyte solute on a pure solvent will be examined by finding the freezing point depression of a cyclohexane unknown so solution. The freezing point of a pure sample of cyclohexane will be determined. Introducing a nonelectrolyte solute should in solution formed to freeze at a lower temperature. This temperature will then be experimentally determined As stated the The molality of the solution dependent on the quantity of solute particles present in solution. (moles solute particles per kilogram of solvent) is directly proportional to the change in temperature between the freezing point of the pure solvent and that of the ATfss Kim The molal freezing point depression constant, K for cyclohexane is equal to 20.00 cfmolal By experimentally determining the freezing point depression of a ohexanel unknown solution, the molal concentration is easily calculated. In turn the molar mass of the unknown olute can be calculated as well Chemistry 132 EXPERIMENT23

Please help me solve for the molar mass of sample 1 and 2 and the molecular formula

Experiment 1084-03: Determination of M.M. by Freezing Point Depres sion Measure the freezing-point depressions solute whose empirical formula is known Pool the class hat occur with solutions containing mass of the solute and its Background Information However, as soon as the solution begins to freeze, Adding a solute to a pure, non-volatile lquid ature will remain constant untl all the (solvent) lowers the vapor pressure of the solvent raises its boiling point, and lowers (depresses) its freezing point. The extent to which these are aftected depends on the relative number of solute particlos in the solution. Properties of solutions that depend only on the number of solute molecules in a given quantity of solvent are called colligative properties. Such cyclohexane turns into solid. Once the substance again. The freezing point is the temperature where it stays constant, Le. the liquld phase and the sold phase of the cyclohexane coexists (Figure 3-1) boiling point elevation and osmotic pressure. They do NOT dopend on the nature of the solute, but DO depend on the nature of the solvent. As a consequence, colligative properties can be used to determine an unknown molar mass for a solute. In this experiment, we will be using cyclohexane (d " 0.779 gmL, K, . 20.5 oC/m) as the solvent to determine the molar mass of a white solid with an empirical formula of C,HC. The freezing point depression, Î7, is the difference between the freezing point of the pure solvent, TP, and the freezing point of the solution (solvent and solute). T As mentioned above, thits quantity is proportional only to the molal concentration of the solute, m Figure 3-1. Cooling Curves The proportionality constant, K, is called the freezing-point-depression constant. The K value of 20.5 C/m for Cyclohexane is one of the largest values one can find. Molal concentration is defined as the number of moles of substance in 1 kg of solvent. Now we can calculate the molar mass of a solute from ATã K,, the mass of the The freezing point of a solution in cyclohexane can be obtained in a similar way. Cooling results in an initial rapid decrease in the temperature until freezing begins as seen in the cooling curve of the pure solvent. However, the temperature of the solution does not remain constant while being frozen. As the cyclohexane in the solution treezes, cyclohexane is taken away from the solution and the concentration of the solute becomes higher In the liquid portion. Thus the molal concentration of the solute increases and the freezing point of the solute and the mass of the solvent. remaining liquid becomes lower. The result is a steadily decreasing freezing temperature until the solution is completely frozen. When all the solution In order to determine ATP, we will measure the freezing point of pure cyclohexane and the freezing point of a solution of the solute in is frozen, the temperature decreases more rapidly To evaluate the freezing point of pure cyclohexame, you will cool a sample of this substance, and measure the temperature as a The freezing point of the solution is the temperature at which the solution starts to freeze. portion represents the concentration of the starting function of time. As cooling begins, theOnly at this point, the concentration of the liquid temperature of the cyclohexane decreases rapidly.

Experiment 10 Colligative Properties Introduction A pinch of wa make boiling water hotter. A handful will clear the snow from your steps. Every day we the physical properties of materials around us, but often don't know why we use these particular s or what chemistry is involved. Modifying the temperature of phase changes, by adding a second chemical to the pure solvent are examples of manipulating the Colligative Properties of a solution. this lab, we will study how varying the concentration and type of a salt affects the point of a solution. Colligative properties are properties of a solution containing 2 or more components. Typically, the primary is the solvent and the other called the such as color or hardness, properties depend on the number of particles of astute in the not the identity of the In many cases, the rokute will determine the number of solute particles (degree of ionization, solubility, etc,) in the solvent, but a colligative property does not depend upon the nature of the particles. Two colligative properties commonly encountered are boiling point elevation and freezing point depression (the temperature of of a is lower than of pure solvent). We will be studying boiling point elevation in this Most of the previous work that you have done with solutions has involved units of molarity, M, or moles of solute per liter of solution. Boiling point elevation calculations (as well as those for freezing point depression) or moles of solute per kilogram of solvent. boili change of temperature. When the temperature of a solution changes, the volume of the solution also changes. Since molarity depends on a change in wil the solution's molarity. Molality depends on the mass of the solvent. and thus is independent of temperature. n't Hoff Factor salt into and the of moles inorganic salts in an aque the based on the addition of the number dissociates, the total number of particles in the solution will be of cations and anions produced from the dissociation. Thus; Nacion Na Choaa (2moles of particles) Cacbrs Ca +2ch (3moles of particles) The quantity of particles produced per mole of solute is called the van'tHoff factor and is given the symbol, i, If 1 mole of sodium chloride were completely dissociated, iwould be 2. This theory works very well for dilute solutions and non-electrolytes (i 1, no dissociation at all since nonelectrolytes not ionize), For does not always apply to more concentrated solutions. Thus, when there are a large number of particles in a solution, they have a tendency to recombine as the salt and you don't get 100% dissociation. example, at a NaCl concentration of 0.1M, iwill be 1.81 (90% dissociation) rather than 2 (100%). Boiling Point Elevation solute causes the solution to boil at a higher When a solution is heated, the presence of a non-volatile point of the pure liquid and the temperature than the pure The difference between the boiling boiling point of the solution is: 169