Molar Volume of Gas - Laboratory report PDF

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Molar Volume of Gas Danna Gomes 09/11/2017 Physical Chemistry- CH401

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Abstract: Using hydrochloric acid and magnesium the volume of hydrogen gas was found, calculations were performed and a good result was obtained. I found the molar volume of a gas to be 22.23 L/mol in the first trial and 22.61 L/mol in the second trial at STP using the Combined Gas law and I had an average molar volume of 22.24 L/mol. This lab has a 0.0892% percent error, which is a pretty good number. I was able to use concepts learned in class such as Combined Gas law and how the variables (volume, pressure, temperature) can interfere during the calculations and on the results.

Table 1: Physical and Chemical Properties of the experiment compounds Compound Mol. Weight Boiling Pt. Melting Pt. Density Safety Mg(s) 24.305g/mo 1100 °C 651° C 1.7g/cm3 Highly flammable l HCl(aq) 36.458g/mo -85.1 °C -114.2 °C 1.6g/cm3 Corrosive, liquid cause l burns 712 °C 2.3g/cm3 Gives off MgCl2(aq) 95.205g/mo 1412 °C l irritating or toxic fumes in a fire H2(g) 2.016g/mol -253 °C -259 °C 0.069g/cm3 Extremely flammable

Introduction: Molar volume is by definition the volume occupied by 1 mole of molecules of a substance (or 1 mole of atoms of an element) in the gas phase. As a volume occupied by a gas depends directly on the conditions of temperature and pressure, the molar volume varies as temperature and pressure vary. At STP which is 273K and 1 atm, the volume per mole of molecules of any gas is approximately equal to 22.4 L. We can do an experiment to confirm this, but it is important to note it is necessary to be very careful with this material and procedure so that the margin of error is the smallest possible. The objective of this experiment is to determine the molar mass (or mol) of magnesium through its reaction

3 with hydrochloric acid. The reaction of magnesium with hydrochloric acid produces hydrogen gas, H2. The gaseous hydrogen will be collected and its volume measured, under temperature and pressure known, which will allow to determine the number of moles of H 2 formed and, through reaction stoichiometry, the number of moles of reacted magnesium.

Background: We can use this experiment to show the production of hydrogen gas from equivalents magnesium weights. Gasses are a special phase of matter that can be described if one knows the pressure, temperature, and volume of the system filled by the gas. We can employ various techniques of measuring these variables. Since I will be collecting gas over water I will need to use Dalton’s law of partial pressures to assure a proper reading of gas pressure, because the gas will be saturated with vapor water. The gas pressure, after the reaction, will correspond to the subtraction of the barometric pressure and the water vapor pressure. The Combined Gas law is used to calculate the molar volume of gaseous hydrogen P1V1/T1= P2V2/T2.

Experimental: -Magnesium ribbon (2-0.04g piece) -50ml buret -Gas collection tube (sealed 50ml buret) -100ml graduated cylinder -600ml beaker -6” of string -1M HCl -Rubber stopper -Thermometer

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Procedure: -Record pressure in the building in torr; -Obtain 2 pieces of magnesium ribbon that weigh about 0.04g. Weigh it to the nearest 0.0001g; -Clean the buret and gas collection buret. Fill the measuring buret with DI water; -Measure the dead volume; -Put a 100ml graduated cylinder to an empty 600ml beaker. Fill the graduated cylinder with DI water; -Add 20ml of 1M HCl to gas collection buret; -Fill the gas collection buret to the top with DI water; -Tie magnesium ribbon to the center of the 6” string. Slide the string into the slit in the stopper; -Place the stopper into the top of the gas collection buret so that the magnesium ribbon is in the water inside the gas collection buret; -Cover the hole in the stopper with your finger. Invert the gas collection buret and place it into the filled 100ml graduated cylinder. Do not allow any air bubbles to enter the gas collection tube; -Allow the magnesium to completely react until no magnesium remains; -Adjust the height of the gas collection buret until the water level in the buret is even with the water level in the graduated cylinder. Read the volume of the gas to the nearest 0.1ml; -Read the temperature of the water in the graduated cylinder;

5 -Repeat the experiment. Calculate an average molar volume at STP for hydrogen and the experimental error.

Results: The magnesium ribbon had a good size and silver color, hydrochloric acid was clear. When the reaction was going the magnesium ribbon cut itself in half and was free from the stopper on both trials and the reaction was bubbling a lot. Table 2: Experiment data Barometric Pressure Mass og Mg ribbon Temperature of water and gas Moles of Mg Moles of H2 made Pressure of Hydrogen Volume of gas Gas volume at STP Molar volume of H2(at STP) Average molar volume(True Val.=22.4L) Percent error

Trial 1 742 torr(from barometer) 0.0367g 23 °C

Trial 2 742 torr(from barometer) 0.0382g 23 °C

1.51x10-3 moles 1.51x10-3 moles 720.9 torr 38.38 ml 33.57 ml 22.23 L

1.57x10-3 moles 1.57x10-3 moles 720.9 torr 40.58 ml 35.50 ml 22.61 L

22.42 L

22.42 L

0.0892%

0.0892%

Equations used: Moles Mg=> Mass Mg= (1 mol Mg/molar mass Mg) Because of stoichiometry magnesium and hydrogen has a ratio 1/1 so the number of moles for hydrogen is the same as magnesium. PH2= PB – PH20

=> pressure of hydrogen

P1V1/T1= P2V2/T2

=> Gas volume at STP

Volume of H2 STP/moles of Mg Trial 1+ Trial 2/2

=> Volume of 1 mole STP

=> Average molar volume and compare to true value= 22.4 L

6 Table 3: Vapor Pressure of Water from 0 C to 30 C T C 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

P(torr) 4.6 4.9 5.3 5.7 6.1 6.5 7.0 7.5 8.1 8.6 9.2 9.8 10.5 11.2 12.0 12.8 13.6

T C 17 18 19 20 21 22 23 24 25 26 27 28 29 30

P(torr) 14.5 15.5 16.5 17.5 18.7 19.8 21.1 22.4 23.8 25.2 26.7 28.4 30.0 31.8

Discussion: Looking my results and evaluating the data we can see it was a good final result. But I am not so confident with it because both of my trials had the exact same temperature. Having the right temperature is very important for this experiment because that way we can calculate the pressure of the gas, the gas volume at STP and furthermore, the molar volume at STP which is the purpose of this lab. Also looking at the results we can see the relationships between variables (volume, temperature, pressure) and how dependent they are from each other. We know the volume of a given amount of gas is proportional to the ratio of its temperature and pressure (as the pressure goes up, the temperature goes up and vice-versa). If the gas temperature was higher than 0 °C then that would cause a higher volume of gas to be produced. If evaporations of the water vapor took place and was collected into the hydrogen, then the volume would be higher because more hydrogen would be added. Using a longer magnesium ribbon would cause more gas to produce which means a longer reaction would occur. With all these statements, we can see the Combined Gas law is true for its purpose.

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Conclusion: The purpose of this experiment was determine the molar volume of hydrogen gas and calculate the value for a mole of the gas at STP. The average molar volume was 22.42 L with 22.23 L on the first trial and 22.61 L on the second trial. The purpose was accomplished because the volume of hydrogen was found and the percent error was really small. The results were not expected to obtain no percent error because of the great chance of error and even the smallest changes would affect the outcome. Again, having the same temperature on both trials was not good outcome because that way I could not have a good comparison between both results. In my opinion this can be considered as an error analysis and I should have done a third trial to try to get a different temperature. However, overall I was able to achieve the experimental purpose which was calculating the value for 1 mole of a gas (hydrogen in this lab) at STP and on my results I was very close of the true value 22.4 L.

References: https://pubchem.ncbi.nlm.nih.gov/ http://www.wiredchemist.com/chemistry/data/vapor-pressure...