Question

what problems do you see in assuming that the stearic molecules exist as rectangular prisms. based on molecular structure?

comment on the role. of counting drops of stearic acid solution in the accuracy of determining avogadros number?

the assumption. is made that the stearic acid molecules line up perfectly to cover. the surface. what issues do you see with that assumption and how might. those cause discrepancy in the calculated value for avogrados number

Answer 1

Answer

Asuming stearic acid molecules as rectangular prisms

When you assume the molecules as rectangular prisms, you made assumptions about volume and size of stearic acid that are not true.

• It is true that in alternate conformation the molecule of stearic acid is like a rectangular prism. However, the hydrocarbon chain is not rigid All bonds in the hydrocarbons chain are simple bonds which means that they can rotate and adopt other conforrmations. This could give to the stearic acid a form different of a rectangular prism
• If we consider stearic acid as rectangular blocks we are assumning a volume for stearic acid higher than real volume, because this rectangular prism will contain the molecule of stearic acid plus all the empty space between atoms. This will affect the calculation of number of molecules that forms the monolayer
• In adittion, considering molecules as rectangular blocks implies that there is not any type of interaction between molecules and this is false. Remember that hydrocarbon chains can experiment van der Waals interactions that could made that molecules could be atracted or repulsed between them (this means that the distance between molecules is not a fixed value)

Role of countig drops in accuracy

Counting drops could affect the accuracy because drops has not the same size and volume, this means that the delivered stearic acid is different for each drop.

To improve the accuracy, first it is necessary make a caibration of the Pasteur pipette: determine the number of drops in 1 mL. However, as mentioned before drops has not the same size and volume, them, this calibration will give us an average value of the drops in 1 mL. These uncertainty in volume will affect the accuracy of results

Stearic acid molecules line up perfectly to cover the surface.

This assumption is made on base of that molecules are considered as rectangular blocks. As mentioned above, it is very difficult that all molecules could be line up perctly.

• The bonds in the molecule can rotate => the molecule adopt differents conformations (this means differents shapes) => it difficults the perfect line up
• The intermolecular forces (van der Waals interactions) causes atractions/repulsions between molecules difficulting the aligment of the same in the monolayer

Obviously, the assumption of perfect line up of molecules introduces an error in calculation of Avogadro's number, because the real number of molecules per monolayer will be different of the calculated number of molecules per monolayer