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Get Charged Up

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Magnesium ribbon was reacted with Hydrochloric acid in three different experiments to determine the charge on a metal ion. After running multiple tests in the three different procedures, the Crystallization method proved to be the best method for determining the charge of the metal ion by using mole to mole ratio. Introduction

The objective of the Get Charged Up lab was to determine the charge on a metal ion reacting with HCl by determine the mole ratio in a reaction by determining the amount of excess reactant, amount of product, and amount of hydrogen gas produced as well as finding the best research method to determine the charge. To accomplish this, three different methods were used. Titration was used to react NaOH with HCl to determine the amount of H+ left over after the reaction is complete. Crystallization was used to determine the amount of MgCl₂ made after the reaction is done.

Both of these methods allowed us to calculate the amount of H+ reacting with the amount of Mg. The gas law method and the Ideal Gas Law was also used to determine the mole ratio of magnesium metal to hydrochloric acid by measuring the amount of hydrogen gas based on the pressure and temperature changes during the reaction. With the results from each procedure, graphs were constructed and the most accurate and precise method was discernible. It is predicted that the titration method will be the most effect method of deterring a charge on a metal ion because almost all of the experiment done with titration is done by the experimenter and is the fairly easiest method. A titration is a method of analysis that will allow you to determine the precise endpoint of a reaction and therefore the precise quantity of reactant in the titration flask. Because of this precision, the titration method will be best for determining the charge of a metal ion because it is the most effective process. Method

Three different methods were used to determine which was the most accurate and precise method for determining the charge on a metal ion reacting with HCl. The first method used was the titration method. Titration is the determination of an amount of a substance by reacting the substance with a known amount of another substance. In this lab, NaOH was used to react with HCl to determine the amount of H+ left over after the reaction was complete. Magnesium ribbon was cut into six different masses and placed in six different beakers and allowed to react with 10.0 mL of HCl. An indicator was added to each beaker to determine when there is still H+ present in the solution. After the reaction was completed, each beaker was titrated with sodium hydroxide (NaOH) until an end point was reached (color change from the indicator).

Using the mass of Mg, molar mass of Mg, moles of NaOH, and the end point volume, the moles of H+ reacting with Mg can be calculated. (The actual procedure can be found on pages 39-41 of the lab manual) The second method used to determine the mole ratio between Mg and HCl will be to produce the metal chloride as a solid so it can be weighed. This is called crystallization. Six different beakers were weighed and a piece of magnesium ribbon was cut to various masses (see page 42 of the lab manual) and placed into each beaker. Using a buret, 10.0 mL of HCl was added to each beaker to react along with an indicator (phenolphthalein) and each was allowed to go to completion. After completion, each beaker was placed on a hot plate and brought to a gentle boil. Solutions were boiled until the water and excess HCl had evaporated. The mass of each product (beaker and crystals) was recorded. At the end of the procedure, moles of Mg used in each trial was calculated along with the Moles of Cl in the salt for each trial. (The actual procedure can be found on pages 42-44 of the lab manual)

The Gas Law method was the third method used to determine the amount of hydrogen produced when magnesium metal is reacted with hydrochloric acid. About 0.025 g of magnesium ribbon was placed in a flask after recording its mass. About 80 mL of HCl was obtained and mixed with phenolphthalein. Using a LabQuest unit and Gas Pressure Sensor kit, the HCl mixture was added to the flask with the magnesium ribbon and allowed to react. When reaction was complete, the change of temperature and gas was recorded. This procedure was repeated for different masses of magnesium ribbon (masses found on page 89 of the lab manual). After the completed procedure, moles of H₂ produced in each trial were calculated. (The actual procedure can be found on pages 87-89 of the lab manual)

After each procedure, a graph was created for each set of data to compare and contrast results. It should be noted that not all supplies are listed here as exactly what is needed to perform each method but a complete list can be found in the lab manual preceding each experiment. Pay extreme caution to the handling of HCl as it can cause acid burns on your skin and remove color from your clothing. Should you get any on you or your clothes, rinse with soap and water. Also, when using flask with thermocouple, it will be top heavy and can easily topple over and break. A final precaution, hydrogen gas is being produced during the gas law method and therefore no open flames in the laboratory during that portion of the experiment. Results

The titration method had the best correlation coefficient of the entered data. The crystallization method had the most accurate slope for the determined substances. The ideal gas law method was close to comparing to the titration method but some data points were a little far off from the trend line. All three graphs had a positive correlation in terms of the x and y relationship. Table 1. Crystallization Method

There is a positive correlation between the moles of Mg compared to Moles of Cl in salt. The correlation coefficient of 7964 relates to the accuracy of the data and the slope of the line (1.981x) relates to the mole ratio between the two substances.

Table 2. Ideal Gas Law Method

There is a positive correlation between the moles of Mg compared to Moles of H2 produced. The correlation coefficient of .9956 relates to the accuracy of the data and the slope of the line (0.8167x) relates to the mole ratio between the two substances. Table 3. Titration Method

There is a positive correlation between the moles of Mg compared to Moles of H+ reacting with Mg. The correlation coefficient of .9999 relates to the accuracy of the data and the slope of the line (1.7923x) relates to the mole ratio between the two substances. Discussion

The best method was the crystallization method for determining the charge on a metal ion. The best method is the most accurate method. This was determined by looking at the correlation coefficient, y-intercept, and slope of the line on the respective graph and seeing the trend of positive correlation. The crystallization method had a slope closest to what it was supposed to be (1.981 compared to 2.000) yielding that this method was the most accurate at determining the charge on a metal ion because we knew the mole ratio needed to be 2:1.

Mg (s) + 2H+ (aq) —> Mg²+ (aq) + H₂ (g)
These results disproved my prediction in that I believed the titration method to be the most accurate method at determining the charge of a metal ion. When thinking about my prediction, I ruled out crystallization because I did not think it would be the most accurate due to the different kinds of errors that could possibly occur in comparison to titration or the gas law method. Although crystallization proved to be the most accurate method, it was not the most precise. The most precise method was the titration method because its correlation coefficient was the closest to 1.000 (.9999). This tells us that the data collected was all within about the same range. The lack of crystallization not being the most precise or the lack of the other two methods being such can be attributed to possible error in the laboratory whether it is method error or measurement error.

One of the largest errors that could have occurred with the gas law method was that the equipment was not the best. There were frequent leaks in the thermocouple as well as the stopcock and syringe. Both of these errors could be prevented in the future with better equipment, having grease to essentially “lock-in” the stopper, or running several tests to ensure that the equipment you were using was not faulty. An error that occurred with the titration method was that of over-titration of the solution.

By over-titrating, one cannot be definite at the endpoint volume which was used to determine the moles of NaOH. Not having the correct moles of NaOH you would not have the correct moles of H+ reacting with Mg therefore making your results inaccurate and not having the correct mole ratio to determine the charge of the metal ion. To improve this for the next lab, one could practice titrating different volumes of water until comfortable with titration. The error that could have contributed to all of the three methods of metal ion charge determination was that of the reaction of the magnesium ribbon with the HCl. By not allowing the reaction of the two to go to completion before adding another substance could have skewed the results of each lab.

For future labs including magnesium reacting with HCl, experimenters need to make sure that the reaction has gone to completion each time they are reacted before adding another substance to the mixture. By doing this, you would prevent errors in endpoints. Further research could be done using the gas law method as the devices and equipment we used were not the best quality. By improving quality, we could improve our results which may provide an even better determination of a charge of a metal ion by determining the moles of H₂ produced. Also, once better equipment is acquired, multiple tests should be done to determine the moles of H₂ produced versus the four tests that we ran. Conclusion

This report has discussed the development of a charge on a metal ion based on three different independent experiments. The objectives of this lab were to find the necessary experiment to have the HCl react with magnesium to determine the charge of a metal ion. Both objectives were met. By keeping track of the mass of crystals produced by the crystallization method and calculating the about of moles of Mg and moles of Cl in salt, by comparing the mole ratio of 2:1, the charge of the metal ion was determined. It was a surprising discovery to see that the crystallization method was the most accurate method because of the higher possible error.

References
Malina, Eric. 2013. General Chemistry Lab Manual.

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