Diffusion rate is how fast one substance spontaneously mixes with another. If you open a bottle of perfume in a room, how long does it take to fill the room with odor? If you put a sugar cube in a cup of tea, how long does it take to mix thoroughly. The rate is given in volume/time. For example it may be 10 cubic centimeters per second. The diffusion rate depends on several factors.

Check to see whether the substances diffuse before gathering any data or doing any calculations by putting the two substances in the same container and seeing if they diffuse into each other. Oil, for example, does not diffuse into water. If they do not diffuse, you are done; the diffusion constant is zero.

Find the information you will need to calculate the diffusion rate before you start calculating. Find the radius of the substance that is defusing (the solute). Find the viscosity of the substance it is defusing into (the solvent). Calculate the temperature Kelvin by adding the temperature in the lab (in Celsius) to 273 degrees (to adjust to absolute zero). The temperature should be about 300 degrees. Search a chemistry reference book to find the required molecular radius and viscosity.

Calculate the diffusion rate, "D," using the formula "D = KT/vr" where "K" is a constant equal to 6.67 X 10^-19, "T" is the temperature Kelvin, "v" is viscosity of the solvent, and "r" is the radius of a molecule of the solute. Compare your calculation to this one for dissolving sugar in water: the viscosity of water is 0.001 (in SI units) and the radius of a sucrose molecule is 1/2 nanometer, so D = (6.67 X 10^-19)(300)/(0.001)(0.5 X 10^-9) = 400 cm per second. Be especially careful with the exponents as this can contribute greatly to any error.

Do it again. Double check your starting information and calculate the answer a different way. Put, for example, all the values in scientific notation and collect all the exponents together before combining. Compare answers to make sure that the calculation is correct. Compare the original calculation of the diffusion rate of sugar to this other path: D = (6.67 X 10^-19(300)/(0.001)(0.5 X 10^-9) = (6.67 X 3.0 X 10^-17)/ (1.0 X 5.0 X 10^-13) = 4.00 X 10^4 = 400 cm per second.

Check your calculation by doing a fast experiment to see whether your diffusion constant is anywhere close to the observable diffusion. Put a drop of the solute into a Petri dish of the solvent and time the diffusion. Remember that the diffusion rate is by volume, not by area. Keep the depth of solvent shallow for best results.