Magnesium and Hydrochloric Acid Lab
Jack Murphy Andre Stetser 5210-1
Abstract: By inducing a chemical reaction by subjecting a strip of magnesium to hydrochloric acid, the density of the hydrogen gas emitted can be acquired through a series of calculations. To determine the density, you would first find the moles actually produce by using the Law of Partial Pressure and the Ideal Gas Law. Find the Volume for that amount of moles at STP. Then take that amount of moles and convert to grams. Divide the grams by the Volume at STP and you have the density of H2 gas at STP. Objective: The purpose of this lab is to calculate the number of moles and the density of the hydrogen gas produced when hydrochloric acid and a strip of magnesium react with each other in a controlled environment. The controlled environment is the distilled water because it reduces extraneous reactions as well as allows us to start with no gas and let the H2 rise to the top of the tube to fill it. The lab will utilize the means of the use of equations, such as Daltons Law of Partial Pressure, and the Ideal Gas Law, in order to determine the density. The lab will connect many processes, which may have been separate ideas from before, and force the experimenter to utilize all of the knowledge that he/she possesses in order to calculate the density of the hydrogen gas produced. The calculation of the number of moles of the hydrogen gas in the eudiometer is a secondary goal of this lab and is necessarily in the calculation of the density. Introduction: The simple reaction between 2 separate substances (Mg and HCl) gives the experimenter a solid grasp on how different factors, such as temperature, volume, and density affect each other in this demonstration. This lab teaches the experimenter to perform good observation taking as well as data management. The experimenter learns to keep track of his work and to write it in a clear and concise manner. This work should be given much attention, especially in schools, because it will give the new scientists a solid introduction and viewing of a reaction in progress and an understanding of pressure as well as equilibrium. This should be
given the financial ability to be publicized in many institutions of learning, for it clearly shows an introduction into the world of chemical reactions and utilization of formulas of pressure. Theory: The final result we wish to find is the density of the hydrogen gas. Since the “universally accepted” results of densities of gases are recorded at STP and in g/L, we will have to convert our units. It is crucial that the experimenter must have a full grasp on many concepts pertaining to mass, volume, etc. in order to fully achieve the accurate data. The universally “accepted” results would be the discernment of the number of moles of hydrogen gas in the eudiometer that is within a 10% error yield of the theoretical yield which can be produced if all conditions are right and a density of H2 gas at STP of .0899g/L. Procedure: Equipment Beaker
Eudiometer
Hydrochloric Acid
Magnesium Strip
Rubber Stopper/ String
Squeeze Bottle
Stand with Clamp
Schematic
Method First, we received a strip of Mg and weighed and recorded its weight in grams. We then tied the magnesium strip onto the stopper using the little piece of string. We filled up the beaker about 2/3 full with tap water. We measured the tap water, and then recorded the temperature of it using a thermometer. We then acquired the distilled water, placed it into the squeeze bottle for transportation, and used the thermometer to record the temperature of it. After this was done, we firmly attached the eudiometer on the ring clasp and stand, and poured about 10mL of 6M hydrochloric acid into it. Using the squeeze bottle, we filled the rest of the eudiometer with distilled water to the very lip so that when the stopper is placed in, water will be pushed out. This is done so that no outside gas is recorded in our calculation. After this was done, we recorded the temperature of the mixture in the eudiometer using the thermometer. This will be the temperature to base the water vapor pressure from. Next we plugged the eudiometer with the rubber stopper and string and made sure the Mg was inside the tube. Then we loosened the grip of the ring, placed our thumb over the hold of the rubber stopper, flipped the eudiometer upside down, and placed the top with the magnesium strip tied to the stopper into a beaker filled with tap water and removed our thumb. As we waited for the magnesium strip to dissipate, we recorded the pressure and temperature of the room. When the magnesium strip finally fully disappeared, we recorded the volume of the gas left in the eudiometer after flicking the tube to get all the bubbles off of the sides (which is impossible).
Results: 1. Table of all the Starting Data Mass of Mg strip (in grams) .06 .002mol *A mol
Volume of HCl (in mL) 10
Tap Water Volume (in mL) 1300
Tap Water Temp. (in °C) 20.3
Distilled H2O Temp. (in °C) 19.6
Eudiometer Temp. (in °C)
Pressure Temp. of of Room room (in Hg) (in °C)
20.3 293.3K *B
31.01
21 294K
Final Volume of H2 Gas (in mL) 55.8
2. Table of the Pressures in the Experiment Barometric Pressure of Room
Water Vapor Pressure
Pressure of H2 Gas
31.01 in Hg
17.5 mm Hg
-----------------------------------
1.04 atm *C
2.3 X 10-3 atm *F
1.01 atm *H
* any letter with an asterisk before it indicates a more exact value is kept track of in the calculator for future calculations. These letters are in place for the whole exact number and may be used in later calculations. 3. Table for the Ideal Gas Law Pressure of H2
Volume of H2
1.01 atm *H
0.0558 L *E
Number of Moles of H2 2.34 X 10-3 same as Mg *A
Universal Gas Constant 0.082058
Temperature of H2 293.3 K
Universal Gas Constant 0.082058
Temperature of H2 273.3
4. Table for the Calculation of the Density Pressure of H2 1.01 atm same as *H
Volume of H2 At STP 5.26 X 10-2 L *I
Number of Moles of H2 2.34 X 10-3
5. Table for the Theoretical Yield, The Experimental Yield, and the Percent Error Theoretical Yield
Experimental Yield
Percent Error
2.47 X 10-3 *A
2.34 X 10-3 *H
4.83%
6. Table for the Calculation of the Mass of the Hydrogen Mass (in g)
Volume (in L)
Molecular Mass
4.74 X 10-3 g *J
5.26 X 10-2 L *I
2.01588 2(1.00794)
Interpretation of Results: Throughout the course of this experiment, there were many different factors and equations which have been used to determine density of the hydrogen gas. The data present in Table 1 is self-explanatory, and shows the preliminary data which is acquired during the course of the experiment. Table 2 requires the production of the pressure of the hydrogen gas by subtracting vapor pressure of water from the room. A miscalculation could be created if you do not convert the units to atmospheres. The following equation should look as such. Partial Pressure Law = Ptotal = Pvapor + PH2 Solve for PH PH = Ptotal - Pvapor (31.01 in Hg X
Ptotal
) - (17.5 mm Hg X
) = *G atm
-
= PH
Pvapor
Take note that the conversion factor from in Hg to atm is the multiplication of the starting factor with 0.0334; the conversion factor from mm Hg to atm is the division of the starting factor by 760. Take heed that when the table states approximately, this is saying that for writing purposes significant figures have been used in order to make this fit on the computer. DO NOT use significant figures until the very last calculation has been acquired. Next, we have to find the number of moles present of hydrogen gas. The format should use the Ideal Gas Law to acquire the number: PV=nRT Make sure that the starting equation is correct. When finding the number of moles, the starting equation should be as such: n=
The first step in using the Ideal Gas Law is to convert all the numbers to their respective unit: 55.8 mL X 1000 = 0.0558 L;
20.3 °C + 273 = 293.3 K
Make sure that R is the Universal Gas Constant:
0.082058
Now that we have acquired all the needed data in their respective unit form, we can now plug and chug.
n= n= 2.35 X 10-3 *H
This represents the number of moles of hydrogen gas which are actually contained in the eudiometer. This will be used to represent the experimental yield. The theoretical yield V=
V=
Determine Volume at STP V=5.25 X 10-2 *I Volume at STP
Now we must find the experimental yield through molar ratios. Mg + 2HCl MgCl2 + H2 Mg:H2 is a 1:1 ratio Next we must determine the number of moles of Magnesium, which will be equal to the number of moles of H2 gas. .06 g Mg X
= 2.47 X 10-3 mol *A
X 100% = 4.83% The percent error // We calculated that we were missing about 2.8 mL of H2 which could easily be explained by the extraneous bubbles stuck to the sides of the tubes. Now, we need to find the amount of grams of H2 for the density. M = mol(molar weigh) *H x (2(1.00794)) = 4.98 X 10-3 g *J
Now we must determine the density which is mass per unit volume D=
D = =8.99 X 10-2
This is extremely important because this precisely matches the universally accepted results The density of the gas in our experiment without STP is: D = = 8.48 X 10-2 g/L of H2 gas at 293.3K and 1.01 atm of H2
One major keynote that you want to be aware of is to never use significant figures until the very end of the calculation. If you have put any miscalculations or errors, make sure to record them. Always be careful when attaching the magnesium strip on the string to the stopper, for if the magnesium floats up to the top of the eudiometer and does not stay attached to the stopper, you will have to redo the experiment. Make sure that the stopper is completely on and secure before flipping the eudiometer upside-down into the distilled water, for if the plug comes out and releases the tube’s contents into the distilled water, you will have to redo the experiment. Always make sure that when you are recording a calculation to convert your data to the right unit (i.e. mL to L, °C to K, etc.). If you are concerned about any of the calculations concerning the number of moles, make sure to note that because it is universally accepted that the pressure inside of the eudiometer is equal to the external pressure, it is possible to determine the number of moles present in the tube after using the partial pressure of gasses to eliminate the pressure of the water vapor. This means that the pressure of the hydrogen and the water vapor combined is
equal to the barometric pressure outside the eudiometer (the pressure of the room) minus the water vapor pressure. When undertaking the process of filling the eudiometer containing the hydrochloric acid with water, make sure that you are filling it with distilled water, NOT the tap water. Distilled water diminishes all chances of having extraneous reactions with the hydrochloric acid that are not desired. This will be a huge difference in calculations, which you will discern at the end of the experiment. Conclusion: Through this experiment, we have tested the process in which the density of a gas can be acquired through the undertaking of a chemical reaction, in this case, one between hydrochloric acid and magnesium. The final density that we acquired is 8.99 X 10-2 . This is exactly the accepted result, which shows our lab was a success. The process of this experiment may be used in the future for testing chemical reactions on a much larger scale. The number of moles produced was also within the universally “accepted” ten percent error margin. The same process can be done for pharmaceutical or nuclear testing, and while the actual variables and elements used in the experiment may change, the formulas and equations may not.
