Volatile liquid, molar mass

The ideal gas law offers a simple approach to the experimental determination of the molar mass of a gas. Indeed, this approach can be applied to volatile liquids like acetone (Example 5.4). All you need to know is the mass of a sample confined to a container of fixed volume at a particular temperature and pressure. 

At a particular location in a distillation column, where the temperature is 350 K and the pressure 500 m Hg, the tnol fraction of the more volatile component in the vapour is 0.7 at the interface with the liquid and 0.5 in the bulk of the vapour. The molar latent heat of the more volatile component is 1.5 times that of the less volatile. Calculate the mass transferrates (kmol m s-11 of the two components. The resistance to mass transfer in the vapour may be considered to lie in a stagnant film of thickness 0.5 mm at the interface. The diffusivity in the vapour mixture is 2 x )() ° mV. ... 

Alcohols with low molar masses are liquids, and alcohols have much lower vapor pressures than do hydrocarbons with approximately the same molar mass. For example, ethanol is a liquid at room temperature, but butane, which has a higher molar mass than ethanol, is a gas. The relatively low volatility of alcohols is a sign of the strength of hydrogen bonds. The ability of alcohols to form hydrogen bonds also accounts for the solubility in water of alcohols with low molar mass. 

A student performed an experiment to determine the molar mass (MM) of a volatile liquid by determining its vapor density. She placed a small amount of a volatile liquid into a flask of known volume then covered the flask with aluminum foil that had been punctured with a small pin. She then placed the apparatus into a boiling water bath and allowed the liquid to vaporize (see figure below). 

B,155,1 m Z. Shakhashiri, "Determination of the Molecular Mass of a Volatile Liquid," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 1 (The University of Wisconsin Press, Madison, 1983) pp. 51-54. A boiling-water bath is used to heat a volatile liquid until it is vaporized, completely filling an Erlenmeyer flask covered by aluminum foil with a pinhole orifice. From atmospheric pressure, bath temperature, volume of the flask, and mass of recondensed vapor, the molar mass of the volatile liquid is determined. 

The empirical formula CH2 is not a stable substance. It is necessary to determine the molar mass to determine the molecular formula. If this hydrocarbon were a gas or an easily volatilized liquid, its molar mass could be determined from the density of the gas, as shown in Chapter 5. Supposing such a determination yields a molar mass of about 55 g/mol, what is the molecular formula ... 

You are to determine the molar mass of a volatile liquid using the vapor density method. Assume you have the following equipment available to you ... 

Through another simple rearrangement of the ideal gas law, we can determine the molar mass of an unknown gas or volatile liquid (one that is easily vaporized) ... 

SAMPLE PROBLEM 5.8 Finding the Molar Mass of a Volatile Liquid... 

Separate species in the liquid phase whose molar masses differ by factor of 1000 via bulk CS (example, desalt a solution of proteins, we can separate species in the liquid phase whose molar masses differ by at least 20% via SEC. Use SEC for gas phase separation if the relative volatility (based on vapor pressure of key components) a p is < 1.1. See Distillation Section 4.2. 

Isobutylene, CH2=C(CH3)2, is predominantly produced from cracked petroleum gases, and also, partially, by the dehydration of /-butanol. In industrial polymerizations, isobutylene is liquefied on addition of some diisobutylene, and mixed with about the same quantity of liquid ethylene and then cationically polymerized at -80°C with BF3/H2O. The diisobutylene acts as chain transfer agent and regulates the molar mass. The ethylene does not polymerize under these conditions on the other hand, it dissipates the heat of polymerization by volatilizing. 

FOLLOW-UP PROBLEM 5.8 An en5)ty 149-mL flask weighs 68.322 g before a sample of volatile liquid is added. The flask is then placed in a hot (95.0°C) water bath the barometric pressure is 740. torr. The liquid vaporizes and the gas fiUs the flask. After cooling, flask and condensed liquid together weigh 68.697 g. What is the molar mass of the liquid ... 

Butane has a lower molar mass than hexane, so the dispersion forces are weaker in the former. Consequently, the cohesive forces are comparatively weaker, and so butane is a gas at room temperature, whereas hexane is a volatile liquid. 

Similarly, the molar mass of a volatile liquid can be determined by placing a small volume of it in the bottom of a flask, the mass and volume of which are known. The flask is then immersed in a hot-water bath, causing the volatile liquid to completely evaporate and its vapor to fill the flask. Because the flask is open, some of the excess vapor escapes. When no more vapor escapes, the flask is capped and removed from the water bath. The flask is then weighed to determine the mass of the vapor. (At this point, some or all of the vapor has condensed but the mass remains the same.) The density of the vapor is determined by dividing the mass of the vapor by the volume of the flask. Equation 11.8 is then used to calculate the molar mass of the volatile liquid. 

Two volatile liquids A (molar mass 100 g/mol) and B (molar mass 110 g/ mol) form an ideal solution. At 55°C, A has a vapor pressure of 98 mmHg and B has a vapor pressure of 42 mmHg. A solution is prepared by mixing equal masses of A and B. 

Measured values are frequently substituted into a formula to obtain a value of some other quantity. For example, in the Dumas method for determining the molar mass of a volatile liquid, we collect the vapor produced by the liquid in a known volume V at a known temperature T and a known pressure P. We then determine the mass of this sample. We assume that the vapor obeys the ideal gas law and apply the formula... 

A company has just patented a new synthetic alcohol for alcoholic beverages. The new product is said to have all the pleasant properties associated with ethanol but none of the undesirable effects such as hangover, impairment of motor skills, and risk of addiction. The chemical formula is proprietary. You analyze a sample of the new product by placing a small volume of it in a round-bottomed flask with a volume of 511.0 mL and an evacuated mass of 131.918 g. You submerge the flask in a water bath at 100.0°C and allow the volatile liquid to vaporize. You then cap the flask and remove it from the water bath. You weigh it and determine the mass of the vapor in the flask to be 0.768 g. What is the molar mass of the volatile liquid, and what does it mean with regard to the new product (Assume the pressure in the laboratory is 1 atm.)... 

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