Investigation on finding the Molar Mass of Butane

Aim: To find the molar mass of butane, by finding the number of moles of gas in the container and comparing it to the mass of butane in the container

Theory

Butane (C4H10), also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3. Its only other isomer is methylpropane: CH(CH3)3. It is an organic compound which belongs to the alkane group or organic compounds. It is a highly flammable, colourless and odorless gas at r.

t.p. this, along with the fact that is an easily liquedified gas, is why it is used in lighters as a fuel. Its Relative Molecular Mass is 58.12g, and it is barely soluble in water like most organic compounds: 0.0061 g/100 cm3, at 20 �C.

In the experiment we shall find the mass of butane by calculating the change in mass of the lighter before and after the experiment. We shall find the number of moles in the container by finding the volume, pressure and temperature of butane inside the container, and then use the formula PV = nRT (where P: Absolute Pressure measured in millibars, V: Volume of gas measured in dm3,T: absolute temperature in Kelvin, and R is the universal gas constant, which equals to 83.

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14472 dm3�mbar�K-1�mol-1).

Since we could not measure the temperature or pressure inside the container, we measured the atmospheric pressure and temperature. We assumed that if we waited for enough time, the temperature inside the container will be equal to the atmospheric temperature.

Secondly, the atmospheric temperature = pressure inside the container and the pressure exerted by the column of water. The pressure exerted by the column of water = mgh.

Prediction

I feel that the mass of one mole of butane will be approximately 58, as this is the RMM of Butane (correct to 0 d.p.).

Apparatus

• Lighter (filled with Butane)
• Water
• Trough
• Digital Thermometer [� 0.01�C]
• Top-pan balance [� 0.01g]
• Burette [� 0.05 cm3]
• Pressure Gauge [� 0.5mb]
• Meter Rule [� 0.0005m]
• Table showing Vapour Pressure of water at various Temperatures

Modifications to Method

• We dipped the lighter in water before any of the trials, and then dried it using ethanol in an attempt to reduce the margin of error in the mass reading due to water droplets sticking the lighter.
• The Burette was filled completely with water.
• We did not take the final volume reading when the level of water in the burette was equal level with the water in the trough. Instead, we measured the height of the water column above the level of water in the trough.
• We found the volume of water between the tip of the cylinder and the 0cm3, and included that in our calculations of the volume of water.

Fair Test

After every trial, we dipped the lighter in ethanol and then shook it to dry to ensure that was minimal inaccuracy in the mass reading due to water droplets sticking to the lighter. However, it is impossible to get rid of all the water droplets. Therefore we dipped the lighter in water and dried it using ethanol before any tests in an attempt to ensure that the extra mass (though minimal) due to water droplets on the lighter remained constant throughout the experiment and thus could be ignored.

• We filled the burette completely with water, to ensure that no gases were inside the burette before the trial, which would have affect the pressure readings.
• The pressure inside the container is equal to the pressure due to butane and moisture. Using a table, we shall find the pressure due moisture, and subtract it from our pressure reading to get the pressure due to butane alone.
• We waited a little which after the trail before measuring the air temperature to ensure that the temperature of butane inside the burette was the same.
• We used water inside the burette since butane barely dissolves in water.
• We removed the metal piece at the top of the lighter, as water droplets could easily stick to it. Also, it ensured that the butane could not catch fire.

Safe Test

• We removed the metal piece of the lighter, to ensure the butane could not catch fire.
• We were careful while dipping the lighter in ethanol, not to release butane as ethanol is flammable.

Conclusion

In the experiment, when we opened the nozzle of the lighter, butane escaped from the lighter, causing a decrease in mass of the butane inside the lighter. The butane rose to the top of the burette, since butane is less dense than water. This caused the pressure at the top of the butane to be greater than the atmospheric pressure. Since liquids are mostly incompressible, the increase pressure at the top, created a force on the top of the water column, pushing it down. Hence the volume of gas (butane and vapour) inside the burette would increase, causing the pressure of gas to decrease until it is equal to the atmospheric pressure. This process kept on occuring as more and more bubbles of butane and moisture reached the top, hence at the end of each trial the pressure inside the burrrete can be estimated to be equal to the atmospheric pressure.

From the graph, it has been calculated that the RMM of butane is 53.946�60.2% using the formulae 'RMM = mass/n' and 'pV= nRT'. There are no anomalies, since the graph passes through the error bars of all 4 points.

Evaluation

In the experiment, the accuracy is fairly high as the experimental value for the RMM of butane is 54, while in theory it is 58.12 (percentage error of only 7.089%), which shows that very few systematic error occured during the experiment. This is also shown by the fact that the y-intercept is 0.0125, which is very close to 0, showing an almost completely proportional relationship between mass and n.

Regarding the precision of error, the margin of error has been calculated as 60.2% and is far to high, resulting in a poor precision. However the experiment was more precise than this value of 60.2% shows it to be, because from the graph one can see that all the points lie close to the line of best fit.

In the experiment, various errors could have occured, such as:

Systematic Errors:

• Error in readings of pressure due to high uncertainty.

Random Errors:

• The pressure due to the water column has no been taken into consideration.
• We are assuming that the temperature inside the burette is equal to atmospheric pressure, which may not be true.
• Water droplets may have still stuck to the lighter, causing an error in mass.

To reduce the margin of error we could have:

• Calculated the pressure exerted do to the water column, and subtracted it from the atmospheric pressure to find the pressure inside the container.
• Taken a larger container than a burrette, so that we could release more butane and cause a larger change in mass, so that the uncertainty of 0.01g would have a smaller affect on the margin of error.
• Used a seperate lighter for each trial, eliminating the error caused due to water droplets clinging to the sides of the lighter.