Colligative Property Elevation in Boiling Point To determine the relation relation between the concentration of solute and increase in boiling point of a solution.
Avantika Arun & Monidipa Chowdhury Class XII F 12604, 12612
COLLIGATIVE PROPERTY – PROPERTY – ELEVATION IN BOILING POINT OBJECTIVE To study the effect of addition of a non-volatile solute to a volatile solvent and also to demonstrate that elevation in boiling point depends upon the relative number of moles of the solute and solvent but solvent but doesn’t depend on the nature of the solute. APPARATUS REQUIRED Bunsen burner,tripod stand, wire mesh, 250ml flask, glass stirrer, thermometer, tap water, solutes. THEORY COLLIGATIVE PROPERTIES Colligative properties are those properties of dilute solutions which depend entirely on the number of moles of solute contained in the solution and not on the nature of the solute. It means that two solutions having different components but same mole-fraction of solute can have identical colligative properties. Some of the colligavtive properties are mentioned mentioned below:
I. II. III. IV.
Relative lowering of vapour pressure. Elevation in boiling point. Depression in freezing point . Osmotic pressure.
RELATIVE LOWERING LOWERING OF VAPOUR PRESSURE For an ideal solution, the relative lowering of vapour pressure is equal to the mole fraction of the solute. In a solution , a part of the surface of the liquid is occupied by the solute particles , as such the evaporation of liquid takes place from a lesser liquid area and hence the liquid will now have a lesser vapour pressure as less liquid will change into vapours. Suppose a solution with two components A volatile and B (non-volatile). The vapour pressure is given as Pa ∞ Xa Or Pa = KXa where , K is the constant of proportionality For pure liquid, Xa= 1, then K=P◦a, which is the vapour pressure of pure solvent. Therefore, Pa=P◦aXa
Similarly , if B is also a volatile liquid, the partial vapour pressure Pb of B is proportional to its mole fraction. Hence, Pa∞Xa , Pa=P◦aXa
(1)
Pb∞Xb , Pb=P◦bXb
(2)
This relationship is know as Raoult’s law. It states that for a given solution of two or more miscible volatile liquids, the vapour pressure of each component at a particular temperature is directly proportional to its mole fraction. From equation (1) Pa=P◦aXa Now,suppose B is a non-volatile solute, then then Xa+Xb = 1 , Xa = 1-Xb Pa=P◦aPa=P◦a-P◦aXb P◦a-Pa P◦a-Pa÷P◦a ÷P◦a = Xb Where, P◦a-Pa P◦a-Pa÷P◦a ÷P◦a is the relative lowering in the vapour pressure. ELEVATION IN BOILING POINT The boiling point of a liquid maybe defined as the temperature at which its vapour pressure becomes equal
to the atmospheric pressure. The normal boiling point of pure water is 373 K. The vapour pressure of the solvent in the solution is lowered due to addition of non-volatile solute which leads to an elevation in the boiling point of the solution as now the solution needs more heating to make its vapour pressure equal to the atmospheric pressure. This effect has been illustrated in the vapour pressure curve below: below: The curves AB and CD are the vapour pressure curves for the pure liquid solvent and the solution respectively. At the temperature Tₒ, Tₒ, the vapour pressure of the pure solvent becomes equal to the atmospheric pressure P1 and Tₒ is the boiling point of the solvent. But the vapour pressure of the solution at Tₒ is P which is less than atmospheric pressure P' and therefore it is needed to heat the solvent to a higher temperature say Ti in order that the vapour pressure becomes equal to the atmospheric pressure. Thus Ti is the boiling point of the solution .Thus it is clear that the solution has higher boiling point than the pure solvent or Tb. Evidently Ti-Tb is the elevation in boiling point. Since its magnitude is determined by the vapour pressure lowering the elevation in boiling point is also proportional to solute concentration.
Where K2 is the molal elevation constant or molal ebullioscopic constant. It is quite clear from the above discussion that we can calculate molecular mass of solute by measuring the elevation in boiling point of a solution and elevation in boiling point is a colligative colligative property.
Depression in Freezing Point Freezing point of a substance is the temperature at which solid and liquid phases of the substance co-exist. It is also defined as the temperature at which liquid as well as solid phases have some some vapour pressure. The freezing point of pure water is 273K. If a non-volatile solute is dissolved in a pure solvent, the solvent vapour pressure in the solution is depressed which results in lowering of freezing point.
Where K is the molal depression constant or cryoscopic constant. Thus, it is clear from the discussion that depression in freezing point of solution is a colligative property. Osmotic Pressure The passage of solvent from pure solvent or solution of lower concentration to solution of higher concentration is called Osmosis. Osmotic pressure may be defined as the excess pressure which is to be applied to the solution side to prevent the passage of solvent into it through a semi permeable membrane. Solutions having same osmotic pressure are called isotonic. The solution having higher osmotic pressure than a given given solution is called called hypertonic and if it has lesser osmotic pressure, it is called hypotonic. Osmotic pressure is also a colligative property.
Procedure 1. Set up the apparatus using a 250ml beaker containing 200ml of the experimental solution. 2. Put the beaker on a tripod stand with a wire mesh and use a Bunsen burner to heat the solution
3. A celestial thermometer calibrated up to 110◦ 110 ◦C is immersed in the solution in the the beaker beaker with the help of a clamp stand. 4. The initial temperature taken before starting the experiment was considered as the room temperature. 5. At first, find the boiling point of tap water. This temperature is taken as the standard boiling point of the solution. 6. Now, 6. Now, prepare three different concentrations of NaCl and boil 250ml of each one by one in the beaker. 7. Take the readings of the temperature after every 20 seconds 8. After 90 seconds, take the readings after every 10 seconds in order to easily find out the concurrent result. 9. Repeat the procedure similarly for different concentrations of oxalic acid and take the observations accordingly. RESULT On increasing the concentration, the boiling point of NaCl and Oxalic acid increases.
CONCLUSION The first set of graphs were plotted for temperature of the solution versus time. In the graph for 1M NaCl, there is slow rise in the temperature in the first 60 seconds of the experiment. After that the temperature rises rapidly till 110 seconds. A peak is obtained at 120 seconds. For 2 M NaCl, the initial slow rise is almost same same as for the 1M graph but the peak is obtained much faster in 100 th second. The b.p. is obtained by finding the mean temperature. For 3M NaCl the peak is obtained almost faster at 80 seconds. In the graph for 1M oxalic acid, a slow and steady rise is seen, 80 seconds after which the graph shoots up. The peak is obtained at 100 seconds after which the temperature remains almost constant. For 2 M oxalic acid, the peak is at 90 seconds and then for 3M the peak is at 80 seconds. So it is seen that the time required to attain the peak becomes lesser. lesser. From the two graphs, graphs, it is evident that when the concentration of the solution is increased from 1 M to 3 M, in both cases there is rise in boiling point. The increase in temperature in case of NaCl is 97◦C, 98◦C and 100.2◦C. The first two readings are almost the same
but for 3 M, the reading differs. This difference difference can be attributed to experimental errors as experiment was not conducted in controlled laboratory conditions. Also, the two experiments were not conducted simultaneously and due to non-availability of distilled water, tap water was also used. Moreover, due to prolonged heating, some some of the solution evaporates evaporates bringing about a change change in the actual concentrations. concentrations. So it can be suggested that increase in b.p is dependent only on the number of moles of solute and not on the nature of the solute whether it is NaCl or oxalic acid. It is proved that when a non-volatile solute is added to a volatile solvent, the b.p. of the solvent increases. Also, this increase in b.p is not dependent on the nature of the solute but depends only on the number of moles of the solute. Thus, this elevation in b.p. is a colligative property.
