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Molar Volume of a Gas Essay

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In this lab I am going to find out the volume of one mole of hydrogen gas at room temperature and atmospheric pressure. The room pressure only slightly differs from the standard, but can be taken into consideration when calculating the results. The molar volume is 22.41 liters per mole at STP (Standard pressure), in other words, at zero degrees centigrade.

Figure 3.1 (the experiment set up)


1. Set up all equipment.

2. Cut a piece of Magnesium ribbon about 20cm in length.

3. Calculate the weight of the ribbon from the weight of a 1 m long ribbon.

4. Measure 1.0 M Hydrochloric acid to a volume of 25-30ml.

5. Pour the HCl to the reaction flask.

6. Add the Mg ribbon to the reaction flask and secure the mouth of the flask as fast as possible with a hose. Make sure that the hydrogen gas cannot escape from the flask.

7. Follow the temperature

8. Collect the gas until no further reaction is observed in the reaction flask.

9. Carefully remove the gas collection flask so that no gas escapes from the flask.

10. Light the gas.

11. Determine the volume of the gas.

12. Calculate the molar volume of H2 gas at room temperature.

a) theoretical value from Vm =22.44 l/mol at STP.

b) experimental value from your data.


* 2 flasks (volume at least 600ml)

* large container (volume at least 3,5 l)

* Magnesium ribbon

* 30ml of Hydrochloric acid

* thermometer

* a hose (to cover the reaction flask)


* Magnesium ribbon was a little oxidized for it had lost some of its shine.

* When the collection flask was turned around and placed in the water filled container, there were tiny air bubbles on the inside walls of the flask.

* When Magnesium ribbon was added, it began to corrose in the HCl

* Instantly after adding the Mg ribbon to the hydrochloric acid, temperature in the reaction flask started to rise as a chemical reaction took place in the flask.

* Moisture and (fog) blocked the view of the reaction

* Immediately after adding the Mg ribbon, hydrogen bubbled to the collection flask

* Hydrogen bubbled to the flask for about a minute,stopping soon after the ribbon had corrosed in to the HCl.

* There was a hissing sound as the chemical reaction occurred in the reaction flask and the gas flowed to collection flask through a pipe.

* The temperature in flask A rose quickly by a few degrees celsius and then stayed nearly constant for the 15 minutes the temperature was measured.

* The temperature did not change in flask B;however, it cannot be stated for sure as it was only compared with the temperature of flask A.

* The temperature in flask A rose very quickly as the reaction started

* The amount of hydrogen produced from the reaction was large for large bubbles of gas rose to beaker B within few seconds after starting the experiment

* The volume of the reaction flask was not measured, but it is close to the volume of the other flask (629ml)

* After the reaction, when lighting the hydrogen gas, collection beaker made a popping sound but the reaction flask actually burned and formed a thin flame.

* The flame from the reaction flask gave out a lot of heat, which was not noticed when lighting the gas in the collection flask.

Theoretical = 22.5 l/mol

Vm= 22.41

0.36 % error

The molar volume was 0.36% too large. The error can come from

– water vapour in the collection flask

– 10cm water below the glass

– air in the collection flask

– temperature rise in the reaction flask

– water vapour in the reaction flask


This experiment had many error-causing factors, which probably influenced the results. Overall, the calculations showed that very little error (0.36% error) which made me a little skeptical about the results. Such a small error percentage was not expected.

To point out a few mistakes, I did not measure the difference in height of the water level from the surface of the water-filled container. This would have allowed further investigation about the volume of hydrogen in the flask. Also, the exact volume of the reaction flask was not measured. It was only stated by eye that the flasks looked to be the same size.

Other than that, the experiment was performed well. It was made sure that none or very little air was left in the collection flask when turning it around and placing it in the water filled container. There were a few tiny air bubbles on the walls; however, the air in the reaction flask was more likely to influence the result. Having a lab partner helped with managing time, for one was able to follow the temperature while the other checked the time. We made sure that we had read the instructions carefully and thought twice before deciding what methods to use when, for example, turning the flask upside down in the water.

Conclusion: In conclusion, the experiment turned out some successful results. When magnesium reacted with the hydrochloric acid, hydrogen was released into the flask from where it flowed through a pipe to the collection flask. The molar volume of H2 gas at room temperature is 22.79 l/mol, which is also the rate for the hydrogen gas in the collection flask. The amount of H2 gas in the flask is impossible to calculate for the amount of water vapour and air was measured when performing the experiment. The experimental value turned out unexpectably very small. 0.36% error in the experiment seems very small, unless there has been some unnoticed mistakes that have influenced the experimental value.

The theoretical value is 22.5 l/mol.

Temperature changes during the experiment turned out some interesting results, for the temperature seems to start falling soon after the chemical reaction has ended, yet it begins to rise a little after a few minutes and stays constant for a long time before starting to fall (figure 3.2). From the information gained during this experiment, it is difficult to state why this happened; therefore, some extra research should take place if performing the experiment again.

Improved investigation: For further investigation, temperature should be measured for longer than 15 minutes in order to find out the rate the temperature is going to fall in a closed flask. The distance between water surface in the container and the surface in the flask should also be measured. For more accurate results, factors such as air and water vapour in the flask should be taken into consideration when calculating the final values. Advisable would be to do some research on why the temperature changed the way it did in this experiment.

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