Research question - How many molecules are there in a liquid drop?

International Baccalaureate Chemistry

Chemistry Lab Report - Molecules in a drop of a liquid

Variables -

Independent variable - The nature of the liquid drop.

Dependent variable - Mass of liquid drop.

Constants -

Concentration of the liquids
The volume of a drop
Temperature of the liquids

Hypotheses and prediction - The heavier the liquid used i.e. a liquid with a high relative molar mass, the more the number of molecules per drop. I predict this as the RMM (relative molar mass) is the measure of the mass of molecules that make up a mole of a substance, and hence the higher the mass is, the more the number of molecules there have to be. Thus, the liquid would have more number of molecules per unit volume as compared to one with a lower RMM, keeping in mind the same concentration is taken.

Apparatus -

Measuring scale, in grams (± 0.01 g)
Dropper
Beaker, 50 ml
Distilled water
Glycerine
Ethanol
Ethylene glycol
Tissue paper

Methodology -

We collected the apparatus needed and measured the mass of the 50 ml beaker. We called it m₁.
Using a dropper, we put 20 drops of water in the beaker. We measured the mass of the beaker + water, and called it m₂. The mass of the 20 drops of water was found by subtracting m₁ from m₂. The answer was divided by 20 to find out the mass of one drop of water.
We repeated step 2, with water, using 40, 60, 80 and 100 drops. This made the experiment more accurate i.e. gave a more precise mass of the water drop.
then, we repeated steps 3 and 4 with the three other liquids - ethanol, glycerine and ethylene glycol.
Values were noted down. Further calculations were made using the mole equation -

Number of moles =

And, also using Avogadro's constant, where the number of molecules in one mole of a substance is 6.023 × 10²³.

Controlling, varying and monitoring the variables -

The independent variable was varied by using not one, but four different types of liquid. These were - distilled water, glycerine, ethanol and ethylene glycol. These liquids have different relative molecular masses.
The change of the dependent variable were monitored by using a measuring scale to observe the change in the masses of the same number of drops when different liquids were tried.
The controlled variables were kept constant:-

All the four liquids had the same concentration of 1 mol/dm³. This was necessary as a change in the concentration produces a change in the number of moles of the liquid in the drop.
The drops were all of the same sizes, and hence of the same volume. the volume was kept constant by using the same dropper for each trial, and furthermore, by applying the same pressure (from the fingers) to the bulb of the dropper.
The temperature of the liquid was necessary to keep constant as even trivial changes in temperatures can make a liquid expand or contract, changing its volume. The experiment was carried out at room temperature, for all trials. The temperature of the surroundings was unchanged throughout the experiment i.e. the temperature of the air conditioner was not altered.

Collecting relevant and sufficient data -

Before the experiment, several trials were executed in order to get a gist of the experiment and recognize and amend any errors. Examples of errors include applying different amounts of pressure on the dropper bulb, giving us drops of different volumes. We also noticed that sometimes, more or less drops were added than needed, due to not observing well or counting the number of drops being put into the beaker carefully. We corrected this by paying more attention to the number of drops being put into the beaker. These errors were made right and taking trials before the experiment ensured we had a more precise, accurate and relevant experiment. We also decided to take the mass as the dependent variable, instead of volume, as we were provided with a measuring scale which was much more accurate (± 0.01 g) as compared to even the most accurate measuring cylinder (10 ml, ± 0.1 ml). This reduced the overall uncertainty of the equipment used and hence the general error of the experiment, and made the data more relevant and certain. On the other hand, it was made sure sufficient data was collected as we took five different trials (20, 40, 60, 80 and 100 drops) for each of the four liquids, just to average it down and get the mass of one drop (for each liquid). Furthermore, we measured the masses of high numbers of drops ex:- 60, 80, 100 drops etc. as the higher the number of drops, the lesser the error uncertainty.

Data Collection -

Table 1 - The raw data which was collected showing the mass of different number of drops of the four liquids.

Table 2 - Table showing calculated averages and the final average mass of the four different liquids, along with the number of moles and the number of molecules.

This is a preview of the whole essay

Data Collection -

Table 1 - The raw data which was collected showing the mass of different number of drops of the four liquids.

Table 2 - Table showing calculated averages and the final average mass of the four different liquids, along with the number of moles and the number of molecules.

The standard deviations of the averages of each set of drops has not been calculated, as it isn't the final value needed (i.e. the average mass of one drop is the final value needed). I have rounded off those averages to three decimal places (instead of one) as the values are very small. The average mass of one drop has been rounded off to the same number of places as the standard deviation, that is two significant figures.

The calculations are shown on the following page.

Calculations -

The averages have been calculated the following way:-

For example, taking the values for water =

= = 0.0634 = 6.3 × 10¯² (to one dp)

The standard deviation for the averages have been found out in the following way:-

First the average of the values have been found. Taking the example of the values of water the average is 6.3 × 10¯² g (0.0634 g).
Then, the difference between each reading and the average was found. That is:

0.058 - 0.0634 = -0.0054

0.059 - 0.0634 = -0.0044

0.065 - 0.0634 = 0.0016

0.067 - 0.0634 = 0.0036

0.068 - 0.0634 = 0.0046

Next, these differences were squared (in order to remove any negative signs):

(-0.0054)² = 2.916 × 10¯⁵

(-0.0044)² = 1.936 × 10¯⁵

(0.0016)² = 2.56 × 10¯⁶

(0.0036)² = 1.296 × 10¯⁵

(0.0046)² = 2.116 × 10¯⁵

These squares were then added, and the sum was divided by (n - 1), where "n" is the number of values.

= 2.13 × 10¯⁵

Finally, the square root of this number gives the standard deviation of the average:

= ± 4.615 × 10¯³

However, this value is always rounded off to one significant figure (hence, so is the average value) giving - ± 0.2 s.

This method was used to get the standard deviation of the rest of the four averages as well.

The number of moles of the liquid contained in the drop was calculated by the formula = Number of moles = .

The relative molar masses of the four liquids were taken from literature values -

Water - 18 ; Glycerine - 92 ; Ethanol - 46 and Ethylene Glycol - 62. (www.wikipedia.com)

The number of molecules present in the drop was found out by using Avogadro's formula which states - Number of molecules = Number of moles of the substance × (6.023 × 10²³)

Data processing -

Graph 1 -

This graph shows us two things - the mass of the liquid drop as well as the number of molecules each drop contains - of four different liquids, which are placed on the X axis. Comparing this graph, and literature values, we can see there is an indirect relationship between the mass of the drop and the number of molecules. This relationship is most importantly affected by the relative molar mass (RMM) of the liquid. A higher RMM means a lesser number of moles in a given volume, as is seen in the case of glycerine, where the number of molecules is seen to be relatively lesser when compared to its mass; and other values. This means that glycerine's molecules are heavy, large or more dense. Whereas in the case of water, the number of molecules is seen to be much higher as compared its mass - which suggests that water has a lower RMM, relatively, and hence is "lighter", or smaller, on the whole.

This graph also shows us anomalous results regarding the mass of the ethylene glycol drop. Technically, the ethylene glycol drop should have a greater mass as when compared to ethanol, as it has a greater RMM (value got from literature data) and a lesser number of molecules. This could have been due to errors in the volume of the liquid drop (for example), which have been explained in the evaluation.

Conclusion -

Thus, we can conclude by stating that the hypothesis has been proved wrong i.e. as the relative molecular mass of a liquid increases, or the mass of the liquid drop increases, the number of molecules it contains decreases. This is because the relative molar mass is a measure of the mass of one mole of a substance (relative to 1/12 of the mass of carbon 12), and one mole of any substance consists of the same number of molecules - 6.023 × 10²³. However, one mole of a substance may differ in mass from one mole of another substance. This is solely because of the mass of the particles contained in that one mole of the substance. A compound which has i) many atoms ii) heavy atoms (in one molecule), will have a higher relative molar mass than a molecule of a compound which has lesser atoms or lighter ones (or both). In this experiment, we are not measuring the number of molecules in one mole of these for substances, but in one drop. hence, the volume remains constant here. Thus, the only way a drop of a substance (of the same volume as the other three drops) will have more number of molecules than any other will be by the liquid having a lower RMM, so that more number of particles would fit in that drop. Taking the example of water, its mass is relatively lower as compared to the number of molecules it contains. This simply suggests that a water molecule will either have lesser atoms, or lighter atoms, or both. On the other hand, the molecule of glycerine is fairly heavy, with an RMM of 92 (whereas the RMM of water is 18) and we can see by the graph that the number of molecules it has is relatively lower than that of water's however the mass of the drop is higher than that of water's. This shows that glycerine molecules are heavier than water molecules. Thus, as the mass of the drop increases, the number of molecules it contains decreases.

Errors and observations -

Glycerine doesn't dissolve in water, hence it was difficult to clean the dropper and the measuring cylinder containing it.
Ethanol, being an alcohol, was volatile. Thus, it easily evaporated. This could have been the reason why the mass of the ethanol drop was lower than expected.
Any slight changes in the room temperature would have caused an error to the volume of the drop, since it is so small in volume, ex:- opening of the laboratory door, changing of the temperature of the air conditioner, opening of the windows. However, it was made sure as far as it could to avoid these changes.
The same dropper was used for each liquid, in order to try to maintain the volume of the drops. This could have resulted in the liquids mixing up, hence altering the mass values.
Minute air particles like dust and dirt could have affected the experiment by changing the mass of the drops.
The readings taken towards the end of the laboratory session were slightly heavier (due to some liquid still remaining in the instrument).
The angle with which the dropper was held made a difference to the size of the drop i.e. if the drop was held vertically, the drops flowed faster and were heavier. Whereas if the dropper was held more horizontally, the speed of the flow of the drops was slower and the size was smaller.
After filling the dropper, the first drops were slightly heavier as compared to the last ones due to the extra pressure being applied to them from the liquid above.
Air bubbles were trapped in the liquids.
Glycerine had the largest and most viscous drops whereas water had the smallest and least viscous drops.
Sometimes, drops were added to a measuring cylinder which already contained drops, intentionally. For example, if a measuring cylinder had 20 drops of water, 20 more drops were added and then the mass for 40 drops of water was measured. In case there were any errors for the first 20 drops, they could have carried on for the next 20 drops.
The liquids could have been slightly impure, as they were all being used for the same experiment (some could have mixed). This would have made a difference to the mass.

Evaluation -

In order to clean the glycerine left from the sides of the dropper etc, a substance which dissolves glycerine could have been used, for example alcohol.
Since ethanol was volatile, the experiment could have been carried out in an area where there was no straight wind. The ethanol bottle could have been kept shut for most of the time, and the masses could have been taken down quickly.
The room temperature could have been well monitored by maintaining a constant temperature (of the air conditioner) and strictly ensuring that the windows or the door weren't opened.
Extra care and hygiene could be taken to ensure that the liquids didn't mix up. This could be done by making sure that attention is paid to the lab while performing it and the volunteers aren't distracted. To make sure the dropper was well cleaned for each trial, liquids which dissolved the liquids being used could have been applied. Another way could be to find another dropper with the exactly same diameter as the one being used. This would decrease the errors by a great deal.
To ensure dust particles didn't fall into the liquids, a conical flask could be used.
After each trial, it should be made sure that the beaker is cleaned well and wiped well too, by tissue paper. In order to ensure that there is no extra liquid remaining, the mass of the beaker could be checked before adding the drops.
One set angle (of the dropper) should be used, for example the dropper could held at approximately 45⁰ to the laboratory surface table for all trials. The pressure applied to the bulb should also be monitored.
When the dropper is full, the experiment could be carried out more slowly and the number of drops be carefully monitored. This would make sure that the size of the drops was not too large, and that the number of drops being added to the beaker were carefully monitored.
To decrease the number of air bubbles, the bottles which contained the four liquids could be shut for most of the time and not moved around much. It should also be made sure that the dropper was full with sufficient liquid so that there would be very less air bubbles, or none at all.
To avoid carry-on errors, each trial could be performed after cleaning the beaker with water and wiping it well with tissue, each time.

These would be the improvements I would add to my experiment in case I perform it again. I would also like to use more, different liquids, in order to get a broader idea of the experiment.