Introduction Accurate measurement of gas density was used for the determination of molecular masses — Physical Chemistry — Thermodynamics Chemistry Question
Determination of molar mass by freezing point depression
Introduction
Accurate measurement of gas density was used for the determination of molecular masses of gaseous compounds in the 19th century. For liquid and solid compounds, however, colligative properties had to be used. Here, freezing point depression will be used to demonstrate how 19th century chemists estimated the molar mass of an unknown compound and determined the molecular formula from a given empirical formula. Freezing point depression also can be used to test Arrhenius’s theory of electrolytic dissociation.
Chemicals and materials
* Ice
* sodium chloride
* unknown compound A (glucose),
* unknown compound B (sucrose),
* wire
Apparatus and glassware
* thermometer or digital temperature sensor (0.1 °C accuracy),
* beaker,
* test tube,
* wire
Mix enough sodium chloride with ice and water in a beaker to bring the temperature of the ice water down to about –8 to –10 °C. Add more ice and salt as necessary to maintain this temperature range.
Add several milliliters of water to the test tube (2~3 cm in diameter). Place a thermometer or digital temperature sensor and a wire bent at one end to form a ring to facilitate mixing. Then immerse the lower half of the test tube assembly into the ice bath and monitor the temperature change for about 10 min while vigorously agitating the water with the wire. The temperature will drop sharply to a point of super–cooling and increase slightly to the freezing point, where the temperature will remain steady. Calibrate the thermometer or the temperature sensor to 0 °C at the freezing point of water.
Prepare 1.00 and 2.00 molal solutions of sodium chloride in water. Determine the freezing point of these solutions following the procedure above. Using the three data points (origin from zero point calibration, 1.00 and 2.00 molal concentration), construct a curve showing freezing point vs. molal concentration. Determine freezing point constant, Kf, of water from the slope and the van’t Hoff i factor for sodium chloride.
– ∆Tf = Kf m I
Dissolve 20 g of unknown compound in 80 g of water. Similarly dissolve 20 g of unknown compound B in 80 g of water. Determine the freezing point depression of these solutions and calculate molality.
From the calculated molality and the number of grams of the compound in 1,000 g of solvent, calculate the molar masses of both compounds.
Elemental analysis showed that the compounds are simple carbohydrates. The mass percentages of C, H, and O (by difference) for both compounds were similar within experimental error (C: 40 ~ 42 %, H: 6 ~ 7 %, O: 51 ~ 54 %). Suggest molecular formulas for compounds A and B.