Interionic Distance

    CAUTION! We shall be using cyclohexane in this experiment. It is highly flammable. Therefore handle cyclohexane in a well-ventilated room or in a fume hood. Do not breathe the vapor. Keep all mixtures containing cyclohexane away from flames. Dispose of leftover cyclohexane in a special container provided. Do NOT put residues down the sink.

    Fortunately, the vapor is not poisonous, and you will be using small quantities of solutions, so there is little danger if you work carefully.

Calibration of a volumetric flask

1. Weigh a clean, dry 10 mL volumetric flask with cap, using the analytical balance. Fill to the calibration with cyclohexane, using a dropper to make the final small addition. (Remember that the bottom of the meniscus should be level with the calibration mark.) Replace the cap to prevent evaporation, and again weigh accurately.

2. Find the temperature of the cyclohexane by inserting a thermometer into the flask. Then pour out the liquid into a small, clean beaker. Allow the flask to drain upside down on a paper towel inside a large beaker.

3. The density of cyclohexane is given by the equation

   ρ_cyclohexane = 0.7743 - 0.00092(t-25) g/cm³

   where the Greek lower-case letter ρ (rho) is the symbol for density, and t is the temperature in °C, Calculate the density of your sample of cyclohexane. Use this value an dthe mass of cyclohexane in the flast to find the accurate volume of your flask. You can use this volume in the next calculation.

Determination of the Density of Sodium Chloride

4. Weigh the empty volumetric flask and its cap again. Add crystals of NaCl (dry ones, of course) until the flask is two-thirds full. Weigh again; calculate the weight of NaCl added.

5. Now add cyclohexane to cover the crystals. Tap the flask gently on the bench while rotating it so that all air bubbles break loose and come to the surface. Fill with cyclohexane to the calibration line and weigh again.

6. From the last two weighings, find the mass of cyclohexane occupying the space in the flask above and around the crystals. You have calculated the density of cyclohexane. Hence, you can find the volume of this space. By subtracting this volume from the total volume of the flask, you can determine the volume of the NaCl. You can then find the density of the NaCl by dividing the mass of your sample by its volume.

7. Now find the volume of the unit cell from the equation given in the discussion part. You will need to know the molar mass of NaCl, to do this calculation. Then find the cube root of the volume (i.e.: the edge length of the unit cell), and from that find the interionic distance.

Finding the Molar Mass of an Unknown Solid

    NaCl is one member of a whole series of salts called alkali metal halides. These salts are composed of cations formed from one of the alikali metals (Li, Na, K, Rb, Cs), and of anions formed from one of the nonmetallic halogens (F, Cl, Br, I). [There is actually one more alkali metal called francium, and one more halogen called astatine, but both are very rare and radioactive, besides.] Many of these salts have the same structure as NaCl. Previous experiments have indicated that many metals also have relatively simple structures as soft iron is BCC and tempered steel is FCC.

    Your instructor will give you a sample of either (a) one of the alkali halides, or (b) a metal. The instructor will also tell you what the crystal structure of the salt or the metal actually is, and the interionic distance (if an alkali halide) or the interatomic distance (if a metal).

    Measure the density of the sample, using either the above procedure or one you learned in the previous semester. If you are given a metal, you will need from 5 to 8 grams. Now combine your knowledge of the interionic or interatomic distance, the structure, and the density of your sample to find the molar mass. Refer back to the previous experiment for details of the structures.

Questions to Think About

1. When salts form by crystallization from solution, they often have inclusion of liquids in them. The liquid is less dense than the crystal. When molten metals solidify, they often have inclusions of air in them. If you measure the density of a large piece of crystal or a metal with this type of inclusion, will this be the true density? If not, will it be higher or lower than the true density? (Hint: last semester, remember the problems involved with determining the density of wood.)
   
   
   
   
2. Can you use the measured density of a substance to tell what particular substance you have?
   
   
   
   
3. How does the interionic distance vary with temperature? How does density vary with temperature?