For liquid of a pure substance, at a given temperature, the vapor pressure decreases with increase of molecular weight. What is the relationship between molecular mass and vapour density? Generally with increase in molecular weight of molecule boiling point increase and hence. Raoult's Law relationship between vapor pressure and concentration of a Fractional vapor pressure lowering can be used to calculate molecular mass. Molar Mass of a Volatile Liquid by the Dumas Method scientists discovered simple relationships among the temperature, pressure, volume, and amounts of gases: We will heat a volatile liquid to evaporate it so that its vapor just fills a flask.
11.5: Vapor Pressure
Eventually, a steady state will be reached in which exactly as many molecules per unit time leave the surface of the liquid vaporize as collide with it condense. At this point, the pressure over the liquid stops increasing and remains constant at a particular value that is characteristic of the liquid at a given temperature. The rate of evaporation depends only on the surface area of the liquid and is essentially constant. The rate of condensation depends on the number of molecules in the vapor phase and increases steadily until it equals the rate of evaporation.
Equilibrium Vapor Pressure Two opposing processes such as evaporation and condensation that occur at the same rate and thus produce no net change in a system, constitute a dynamic equilibrium. In the case of a liquid enclosed in a chamber, the molecules continuously evaporate and condense, but the amounts of liquid and vapor do not change with time. The pressure exerted by a vapor in dynamic equilibrium with a liquid is the equilibrium vapor pressure of the liquid.
If a liquid is in an open container, however, most of the molecules that escape into the vapor phase will not collide with the surface of the liquid and return to the liquid phase. Instead, they will diffuse through the gas phase away from the container, and an equilibrium will never be established.
Volatile liquids have relatively high vapor pressures and tend to evaporate readily; nonvolatile liquids have low vapor pressures and evaporate more slowly.
Vapor Pressure - Chemistry LibreTexts
Thus diethyl ether ethyl etheracetone, and gasoline are volatile, but mercury, ethylene glycol, and motor oil are nonvolatile. The equilibrium vapor pressure of a substance at a particular temperature is a characteristic of the material, like its molecular mass, melting point, and boiling point Table It does not depend on the amount of liquid as long as at least a tiny amount of liquid is present in equilibrium with the vapor.
Molecules that can hydrogen bond, such as ethylene glycol, have a much lower equilibrium vapor pressure than those that cannot, such as octane. The nonlinear increase in vapor pressure with increasing temperature is much steeper than the increase in pressure expected for an ideal gas over the corresponding temperature range.
The temperature dependence is so strong because the vapor pressure depends on the fraction of molecules that have a kinetic energy greater than that needed to escape from the liquid, and this fraction increases exponentially with temperature.
As a result, sealed containers of volatile liquids are potential bombs if subjected to large increases in temperature.Partial Pressures & Vapor Pressure: Crash Course Chemistry #15
Similarly, the small cans 1—5 gallons used to transport gasoline are required by law to have a pop-off pressure release. In this week's experiment, we will determine the pressure, volume, and temperature of a sample of gas our evaporated volatile liquidwhich means that we can solve for the number of moles: But we go one step further, because we will also measure the mass of the evaporated liquid.
Knowing the mass and the number of moles will allow us to compute the molar mass of the liquid: Procedure Overview I won't go over the procedure in step-by-step detail, but I will stress some points of safety and in bold color some places where our procedure differs from that in the lab packet.
The main difference is to measure the volume of the flask last, not first.
Work in pairs no triples. We will heat a volatile liquid to evaporate it so that its vapor just fills a flask whose volume we can measure; we will also know the temperature and pressure of the vapor, and the mass of the vapor.
Determine the mass of flask, boiling stone, and foil cap This is done before adding the liquid so we can weigh the amount of vapor by difference between this and a later measurement; it is done before the first trial only.
Make a small pin-hole in the foil cap. Use narrow-neck mL pyrex flasks only; examine flask for cracks. Add volatile liquid About 5 mL is sufficient. It is not crucial to get exactly 5 mL. Heat to vaporize liquid We will use Bunsen burner flames to heat the water baths. Be careful not to let your flames get too high, because our volatile liquids are flammable.
Diagram of apparatus for our experiment.
Molar Mass of a Volatile Liquid
Photo of apparatus of experiment as done at Rutgers University. Diagram of apparatus as Dumas described it. Put a boiling stone in the water bath too but you don't have to know its mass.