Determine the percentage of oxygen in air experimentally
1. Experiment.
Determination of the presence and proportion of oxygen in air by
combustion of a candle
Method
1. Place a small candle on a plastic lid or any object that can float. Then set up the apparatus
as shown in figure below. Sodium hydroxide is used in order to absorb the carbon dioxide gas produced by a burning candle.
2. Light the candle and place the measuring cylinder over the top. Note the level of sodium
hydroxide solution in the measuring cylinder at the start. A candle will stop burning (go off)
once all the oxygen in the cylinder is used up.
3. When the candle goes off, leave the apparatus to cool to room temperature. The purpose
of cooling is to let the heated and expanded air to return to its normal condition. Then note the level of sodium hydroxide solution in the measuring cylinder.
Observation and findings
Model results
In the experiment, the initial volume of air was found to be 70.5 cm3 and the final volume was 55cm3.
1. To find the volume of oxygen used up to burn the candle (which is practically equal to the
volume of carbon dioxide produced and then absorbed by sodium hydroxide), we subtract the final volume of air from the initial volume
Volume or oxygen used = Initial volume of air – final volume of air
Therefore, the volume of oxygen used for combustion of the candle = 14.7 cm.
Alternatively, the volume of oxygen used up can be calculated by subtracting the initial volume of sodium hydroxide solution from the final volume.
This is due to a number of reasons such as:
1. Not all the carbon dioxide is absorbed by the sodium hydroxide.
2. The candle may go out (stop burning) before all the oxygen is used up due to
accumulation of carbon dioxide in the cylinder.
3. The heating of the air inside the measuring cylinder causes the gases to expand. This is
why it is essential that the gases be allowed to cool to room temperature before reading the level.
Experiment with combustion of copper in air gives the more accurate results than the
combustion of the candle. The copper reacts with oxygen in the air to give copper (II) oxide.
Method.
1. Set up the apparatus as shown in figure below. Syringe A should contain 100 cm of air,
syringe B should be empty.
2. Heat the copper strongly and pass the air from syringe A back and forth (by pushing the
piston of the syringe inward and outward) over the copper turnings a few times. Allow the air to cool and measure the volume of air in syringe A.
3. Repeat the heating and cooling until the volume of air that remains in syringe A is
constant. The copper is heated and cooled several times to ensure that it reacts with all oxygen in the sample of air.
`
Determining the presence and percentage composition of oxygen in air by heating copper
Observations and findings
2. The final volume of air in the syringe, at the end of the experiment, is less than that of the
original volume. This is because oxygen in the original air has combined with copper
Model of result
The presence of carbon dioxide in air
Carbon dioxide is present in air to the extent of 0.03% by volume. The gas is formed during the combustion of all common fuels – wood, coal, coke, natural gas, petrol, diesel, paraffin oil, etc, all of which contain carbon.
However, the amount of carbon dioxide in air remains constant instead of the tremendous
quantities released into the atmosphere. This is because plants take up carbon dioxide. They then convert it into complex starchy compounds during photosynthesis. The gas also dissolves in ocean water and other water bodies.
The presence of carbon dioxide in air can be shown by passing air through a test tube containing some limewater. After a time, the limewater turns milky. This shows the presence of carbon dioxide.
The reaction involved is as follows:
combustion of a candle
Method
1. Place a small candle on a plastic lid or any object that can float. Then set up the apparatus
as shown in figure below. Sodium hydroxide is used in order to absorb the carbon dioxide gas produced by a burning candle.
2. Light the candle and place the measuring cylinder over the top. Note the level of sodium
hydroxide solution in the measuring cylinder at the start. A candle will stop burning (go off)
once all the oxygen in the cylinder is used up.
3. When the candle goes off, leave the apparatus to cool to room temperature. The purpose
of cooling is to let the heated and expanded air to return to its normal condition. Then note the level of sodium hydroxide solution in the measuring cylinder.
Observation and findings
The oxygen in air enclosed in the measuring cylinder is used to burn the candle to produce
carbon dioxide gas. The carbon dioxide so produced dissolves in sodium hydroxide solution.
carbon dioxide gas. The carbon dioxide so produced dissolves in sodium hydroxide solution.
The dissolved carbon dioxide causes the level of sodium hydroxide solution to rise up. The oxygen gas used to burn the candle is practically equal to the amount of carbon dioxide produced. This fact is, therefore, used to calculate the percentage of oxygen in air.
Model results
In the experiment, the initial volume of air was found to be 70.5 cm3 and the final volume was 55cm3.
The percentage of oxygen in the air is calculated in two steps:
1. To find the volume of oxygen used up to burn the candle (which is practically equal to the
volume of carbon dioxide produced and then absorbed by sodium hydroxide), we subtract the final volume of air from the initial volume
Volume or oxygen used = Initial volume of air – final volume of air
Therefore, the volume of oxygen used for combustion of the candle = 14.7 cm.
Alternatively, the volume of oxygen used up can be calculated by subtracting the initial volume of sodium hydroxide solution from the final volume.
That is: Volume of oxygen used = final volume of sodium hydroxide – initial volume of sodium hydroxide = Volume of carbon dioxide dissolved in sodium hydroxide.
2. Therefore, the percentage of oxygen =
In practice, it is difficult to get an accurate result with the above experiment.
This is due to a number of reasons such as:
1. Not all the carbon dioxide is absorbed by the sodium hydroxide.
2. The candle may go out (stop burning) before all the oxygen is used up due to
accumulation of carbon dioxide in the cylinder.
3. The heating of the air inside the measuring cylinder causes the gases to expand. This is
why it is essential that the gases be allowed to cool to room temperature before reading the level.
Experiment with combustion of copper in air gives the more accurate results than the
combustion of the candle. The copper reacts with oxygen in the air to give copper (II) oxide.
2. Experiment.
Determination of the presence and proportion of oxygen in air by the combustion of copper in air
Method.
1. Set up the apparatus as shown in figure below. Syringe A should contain 100 cm of air,
syringe B should be empty.
piston of the syringe inward and outward) over the copper turnings a few times. Allow the air to cool and measure the volume of air in syringe A.
3. Repeat the heating and cooling until the volume of air that remains in syringe A is
constant. The copper is heated and cooled several times to ensure that it reacts with all oxygen in the sample of air.
Determining the presence and percentage composition of oxygen in air by heating copper
Observations and findings
2. The final volume of air in the syringe, at the end of the experiment, is less than that of the
original volume. This is because oxygen in the original air has combined with copper
Model of result
The volume of air in the syringe at different heating and cooling is as shown below:
Initial volume before heating = 100
Volume after first heating and cooling = 82
Volume after third heating and cooling = 79
The volume of oxygen used up = Initial volume of air before cooling - volume of air after the last
heating and cooling
= 100 - 79
= 21
Initial volume before heating = 100
Volume after first heating and cooling = 82
Volume after third heating and cooling = 79
The volume of oxygen used up = Initial volume of air before cooling - volume of air after the last
heating and cooling
= 100 - 79
= 21
The presence of carbon dioxide in air
Carbon dioxide is present in air to the extent of 0.03% by volume. The gas is formed during the combustion of all common fuels – wood, coal, coke, natural gas, petrol, diesel, paraffin oil, etc, all of which contain carbon.
It is breathed out as a waste product of respiration by all animals. All sorts of combustion and burning produce carbon dioxide. The gas produced by all these processes accumulates in air.
However, the amount of carbon dioxide in air remains constant instead of the tremendous
quantities released into the atmosphere. This is because plants take up carbon dioxide. They then convert it into complex starchy compounds during photosynthesis. The gas also dissolves in ocean water and other water bodies.
The presence of carbon dioxide in air can be shown by passing air through a test tube containing some limewater. After a time, the limewater turns milky. This shows the presence of carbon dioxide.
The reaction involved is as follows:
){kind=link}
0 Comments