A Simple Classroom Demonstration of Para and Diamagnetism P. E. Dumas and A. Zozulin Trenton State College Trenton, New Jersey 08625 X\'umen,u~,demonstrations of magnetic properties. for varwus types of subrtnnres, have been developed. Howevrr, most of thrse demonstrations are limited in that they only d i i t i w i s h diamagnetic from paramagnetic materials, or they are difficult to perform effectively in a classroam situation. Our apparatus is easily used in the classroom, and is not only capable of distinguishing between diamagnetic and paramagnetic materials, but can readily distinguish between high and low spin complexes of transition metals, with dramatic results. The apparatus is constructed by gluing a n S-in. strip of balsa wood ('I2 X 'Ig in.) to the pan side of the beam of a hanging pan, triple-beam balance. This serves to lengthen the beam so that the main body of the balance is removed from the magnetic field that is t o be applied. A test tube containing the sample is suspended from the far end of the balsa strip by a Nylon string. A small mirror is mounted above the fulcrum. The weights on the beam are adjusted to bring the rest position indicator slightly below the zero rest position on the balance. A stationary light source, preferably a low power laser, is then focused on the mirror and the position of the reflected light is observed and marked an a convenient wall, screen or blackboard. A permanent magnet with a field strength of about 2-3 kG is moved toward the sample in the suspended test tube. The magnet is positioned so that the sample is exactly between the poles. The position of the magnet should be marked so that identical conditions are duplicated in subsequent measurements. The magnitude and direction of deflection of the light beam will be a measurable function of the magnetic oroverties of the . . . samples. Employing this apparatus we could easily distinguish between high and low spin octahedral complexes of iron and cobalt. Deflections up to40 in. were obtained for high spin complexes..
784
/ Journaiof Chemical Education
A New Equation for the Determination of Molecular Weight and Equilibrium Constant of a Weak Acid Samuel Y. Shen Long Island University Brooklyn, New York When a weak, HX, ionizes, i t releases both H+ and X until an equilibrium is established
HX
ions
= H+ + x-
or,
where K, is the equilibrium constant. and Hz+ is equal t o X-. If the acid were diluted n times, eqn. (1)may he expressed as follows
In the above equation, HX can be expressed as glM, g being the weight of the acid per liter and M its molecular weight. Then, by substitution. cmss multidieation and rearranwment, em. (2) can he rewritten as
Equation (3) shows that once the dilution factor, R and the hydronium-ion concentration of the initial and final solution of HX which contains a definite amount of the acid are known, the molecular weight of this and any monopmtic weak acid can be ealculated. K, of the acid may then he determined by substituting the correct valuesof HI+ and HX into eqn. (1). Equation (3) has been tested with acetic acid a t concentrations between 0.1 and 1.0 M The average calculated value for the molecular weight turns out to be 60.36 and K. is found to be 1.80 X 10-5.
