question_answer 73)

                  In an experiment to estimate the size of a molecule of oleic acid 1 mL of oleic acid is dissolved in 19 mL of alcohol. Then 1 mL of this solution is diluted to 20 mL by adding alcohol. Now 1 drop of this diluted solution is placed on water in a shallow trough. The solution spreads over the surface of water forming one molecule thick layer. Now, lycopodium powder is sprinkled evenly over the film and its diameter is measured. Knowing the valued of the drop and are of the film we can calculate the thickness of the film which will give us the size of oleic acid molecule.                 Read the passage carefully and answer the following questions:                 (a) Why do we dissolve oleic acid in alcohol?                 (b) What is the role of lycopodium power?                 (c) What would be the volume of oleic acid in each mL of solution prepare?                 (d) How will you calculate the volume of n drops of this solution of oleic acid?                 (e) What will be the volume of oleic acid in one drop of this solution?                

Answer:

                  (a) Oleic acid is soluble in alcohol but insoluble in water.                 (b) Lycopodium powder spreads on the entire surface of water. When a drop of the solution of oleic acid in the solution spreads on water surface as it does not dissolve in water. The drop pushed the lycopodium powder particles away and creates a circular area where it falls, thus enabling us to measure the area where oleic acid spreads.                 (c) Since 20 mL (1 mL oleic acid + 19 mL alcohol) contains 1 mL of oleic acid, oleic acid in each mL of the solution \[=\frac{1}{20}\,mL\]. Further, as this 1 mL is diluted to 20 mL by adding alcohol, oleic acid in each mL of prepared solution.                 \[=\frac{\left( \frac{1}{20}mL \right)}{20}\]                 \[=\left( \frac{1}{400} \right)\,mL\]                 (d) By means of a burette (to allow the solution to trickle out in drops so that these can be counted) and measuring their volume with a measuring cylinder, the volume of \[n\] drops can be calculated.                 (e) If \[n\] is the number of drops in \[1\,mL\] of the solutions, volume of oleic acid in one drop \[=\frac{\left[ \frac{1}{400} \right]mL}{n}\]                 \[=\left( \frac{1}{400\,n} \right)mL\]