Question
In this experiment, what is the molecular weight of a given liquid?
Introduction
The most common device used to determine the mass of an unknown sample is called a mass spectrometer which utilizes the interaction of an ionized atom with a known magnetic field. The mass of the sample will determine the rate of curvature and hence the landing place of the sample on a detector. Mass spectrometers are however , a very expensive piece of equipment and the an alternative method may be used to determine mass. We will use the ideal gas law PV =nRT instead. When we rearrange the equation for n we get
n=PV/RT.
We also know that the number of moles of a sample (n) can be determined by using a factor label conversion where we place the given gas weight (m) on top of a factor label conversion and divide this by the known molecular weight of the gas ( MW ). We can simplify this equation to be n = m/MW and can rearrange the equation to get
MW =m/n.
In this lab then we will attempt to determine n the number of moles and then use this derived value to determine MW the molecular weight of the sample.
Procedure
Prepare a water bath using a 600 ml beaker and 200 ml of water on a hot plate. We cannot use a traditional source of heat, a Bunsen burner, due to the risk associated with an open flame and a possibly flammable unknown sample.
Obtain a 500ml reagent bottle and completely CLEAN the container. Fill the clean bottle with water and use a graduated cylinder to determine the total volume of the bottle by transfer. When finished, dry the inside and outside. Any water present on or in the container will have adverse effects on the Molecular Weight determination. Cut a square of aluminum foil that will adequately cover the opening and place a small pinhole opening in the middle of the square. Weigh the empty clean/dry bottle and foil and record this weight from the pan scale (not the analytical scale).
Place 4 ml of an unknown liquid in the bottle, cover and crimp the foil over the opening, Place this in the water bath and allow to heat slowly. Eventually the system will come to the point where the liquid inside the reagent bottle boils. Keep an eye on the sample and when it appears that all the unknown sample has boiled away, remove from the heat with beaker tongs to protect your hands from the heat. Set the bottle on a cool wire mesh and allow to return to room temperature. As it does this, condensation will appear on the inside of the bottle. This is normal. While the bottle is cooling, record the temperature of the water and the barometric pressure for the day. When the bottle has cooled to room temperature, verify that the outside of the bottle is dry, then weigh the bottle, foil and condensate.
Repeat this procedure with another sample to verify findings. If both data sets yield similar results, use the data obtained to find n, the number of moles in the bottle and then use this to determine the value of MW the molecular weight of the gas.
Data
Sample 1 Sample 2
Mass of bottle and cover-
Mass of bottle, cover and condensate-
Temperature of boiling system-
Barometric Pressure-
Volume of Bottle-
Calculations
Mass of Vapor-
Moles of Vapor-
Molar Mass of Vapor-
Analysis Questions
1. Calculate the density of the vapor at the boiling point of water.
2. The fact that we will operate near the boiling point of the liquid will affect how ideal the gas is and hence the calculated value of the molecular weight. What kind of error in value would we expect and justify your answer.
3. If the bottle was not completely clean and dry when we first weighed it, how would that affect our calculated value for molecular weight. Justify your answer.