BACKGROUND INFORMATION Two glass tubes of known dimensions were partly filled with cyclohexane, two more with p-xylene. The ratio of the length to diameter of the tube was 4:1. P-xylene has a higher boiling point and higher molecular mass compared to cyclohexane, so it is anticipated to have a lower diffusivity rate. Table 1 describes physical information: Table 1: Physical Information of Tubes These tubes were partially filled with liquids and placed upright in a well-ventilated, temperature controlled room. The evaporation rate of the volatile compound from the tube was measured by the change in mass liquid as it evaporated. Table 2 contains the initial liquid masses recorded. Table 2: Initial Liquid Masses Diffusion is a function of time, so multiple measurements of the mass of volatile substance remaining in each tube were recorded over a period of several days. Room temperature was also recorded during every data collection.
To model this phenomena, the rate of diffusion described by Fick’s law was used: (1) where JA = molar flux of A in the z direction, relative to the total flow [mol/cm2s] DAB = diffusivity, [cm2/sec] cA = molar concentration of A [mol/cm3] z = direction of concentration gradient [cm] The system must be stagnant for Fick’s law to apply; this assumption can be made for this experiment, as the toluene and cyclohexane are observed in stagnant air. Equation (21.25), page 655, of reference (1) is the equation use for empirical calculations. (2) To describe the empirical nature of the relationship: MA, MB - molecular weights of components A & B, respectively p - pressure, atm. T - temperature, K TcA, TcB - critical temperatures of A & B respectively, K VcA, VcB - critical molar volumes of A & B respectively, cm3/ g mol
RESULTS AND DISCUSSION
CONCLUSTIONS AND RECOMMENDATIONS
The setup of this lab was very straight forward. Once the chemicals were chosen they were carefully poured into the test tubes. P-xylene and cyclohexane were chosen because of their insolubility with water. It was difficult to determine the height with great accuracy using the caliper. The effect is