06/05-W97-Se Instruction sheet 667 4961

7 1 2 Apparatus for molar mass determination, boiling point method (667 4961)

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1 Thermometer connection 2 Drying tube 3 Internal condenser 4 Boiling tube with 2 side tubes 5 Boiling jacket 6 Filler neck 7 Glass stopper

1 Description 2 Scope of supply The apparatus for determining the molar mass, boiling point 1 boiling tube with 2 side tubes method (ebullioscopic method) allows the molar mass of a 1 internal condensor with ST 19/26 dissolved substance to be determined from the amount by which the boiling point of the pure solvent is increased. For an 1 drying tube with ST 7/16 exact measurement of the boiling-point elevation to an 1 glass stopper ST 14.5/23 accuracy of ±0.01 K, the Beckmann thermometer (666 173) or 1 boiling jacket a digital thermometer (666 209 or 666 454 respectively) with an NTC temperature sensor is used. 3 Technical data

Thermometer connection: ST 19/26 Drying tube: ST 7/16 Internal condensor: ST 19/26 Filler neck: ST 14.5/23 Boiling jacket: Glass cylinder with insulation (asbestos free) Safety notes 4 Accessories In order to avoid delays in boiling: 1 Beckmann thermometer 666 173 - Put glass beads, Raschig rings or a platinum tetrahe- or dron into the boiling tube. 1 digital thermometer 666 209 If readily flammable solvents are used: or 666 454 - Remove all naked flames when filling and opening the 1 temperature sensor, NTC, 260 mm 666 2121 apparatus. 1 reducing adapter ST 19/26 to GL18 665 305 1 gas or electric burner

Instruction sheet 667 4961 Page 2/3

5 Putting the apparatus into operation 6 Carrying out the experiment - Accurately measure off approx. 30 ml of solvent using a pipette, and introduce it through the filler neck; immediately seal the filler neck with the glass stopper. - Switch the cooling water supply for the internal condensor on, and heat the boiling jacket uniformly with the gas or electric burner.

0 To attain uniform boiling, see to it that the solvent condensing on the internal condenser flows back continuously. This process can be inhibited by a decrease in gas pressure or by poor insulation of the boiling tube against draught of air. It may be necessary to place a small 5 mica plate on the boiling jacket in order to eliminate draughts. The temperature can be considered constant only if it remains constant to an accuracy of 0.02 K over a period of 3-4 minutes. This should be achieved after 15 minutes at most. - Take the boiling temperature of the pure solvent down. - Press approx. 3-5 g of the substance into a pellet, weigh the pellet on the analytical balance, and throw it in through the filler neck. The pellet must be flung in so that it does not stick in the side tube. - Now determine the boiling point of the solution as described above.

7 Principles If a non-volatile substance is dissolved in a solvent, this leads to a lowering of the vapour pressure or a boiling-point elevation as compared with the pure solvent. This boiling-point elevation depends on the number of dissolved particles but not on the kind of substance dissolved. Thereby the substance to be dissolved must not be volatile with the vapour of the solvent.

For dilute solutions where the number of moles n1 of the substance dissolved can be neglected against the number of - Attach the wire gauze to a stand, and put the boiling jacket moles n0 of the solvent, the change in the boiling temperature on it. is 2 2 - Attach the boiling tube to the stand so that the bottom part R ⋅T0 n1 R ⋅T0 m1 1 of the tube is inside the boiling jacket. ∆T = ⋅ = ⋅ M0 ⋅ ⋅ . Λ0 n0 Λ0 M1 m0 - Insert the internal condenser, and provide it with water inflow and outflow. R: general gas constant, T0: boiling temperature of the solvent (in Kelvin) - To exclude atmospheric moisture, attach the drying tube. Λ0: molar heat of evaporation of the solvent - In order to avoid delays in boiling, put glass beads, Raschig m1: mass of the substance dissolved, m0: mass of the solvent, rings or a platinum tetrahedron into the boiling tube. M1: molar mass of the substance dissolved, - Seal the filler neck with a glass stopper. M0: molar mass of the solvent - Set the Beckmann thermometer to a suitable temperature Thus the boiling-temperature elevation has the form range (see instruction sheet 666 173) and insert it in the R ⋅T 2 ⋅M m 1 upper ground. ∆T = K ⋅c with K = 0 0 and c = 1 ⋅ . E E Λ M m or 0 1 0 - Insert the NTC temperature sensor with the reducing Here kE is the ebullioscopic constant of the solvent and c the adapter in the upper ground. molality of the solution, that is, the ratio of the number of moles of the substance dissolved and the mass of the solvent.

If the ebullioscopic constant of the solvent is known, the molar mass of the substance dissolved can be determined from the boiling-point elevation.

KE m1 M1 = ⋅ ∆T m0

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8 Evaluation example 9 Appendix

Water as a solvent has Ebullioscopic constants KE and boiling temperatures T0 of some liquids: Λ J T = 373.2 K and 0 = 2262.2 . 0 K T M0 g E 0 liquid kg⋅K °C Therefore the ebullioscopic constant of water is mol J 8.312 ⋅ ()373.2 K 2 water 0.512 100.0 K ⋅mol kg ⋅K KE = = 0.512 methanol 0.83 64.7 J mol 2262.2 g ethanol 1.20 78.3 5.74 g of D(+)-glucose dissolved in 30 g of water leads to a acetone 1.72 56.0 boiling-point elevation of ∆T = 0.48 K. diethyl ether 2.16 34.0 kg ⋅K ethyl acetate 2.68 78.0 0.512 KE m1 mol 5.74 g g glacial acetic acid 3.08 118.1 M1 = ⋅ ⋅ = ⋅ = 204 ∆T m0 0.48 K 30 g mol acetic anhydride 3.53 139.6 chloroform 3.85 61.2 trichloroethene 4.43 87.0 nitrobenzene 5.27 210.9

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