CHEM 111 Carbonate Analysis current PDF

Preview

Summary

lab report...

Description

Destiny Cambero CHEM 111 – Farnum MTWR 9:40 AM 08/09/18

Experiment 13: A Carbonate Analysis; Molar Volume of Carbon Dioxide Conclusion: In conclusion, the average molar volume of CO2 in my sample at STP was 19.82 mol/L and the average percent of CaCO3 in my sample was 86.71%. My standard deviation was 2.64, my relative standard deviation was 13.32%, and my %error was 11.52%. Since my data was above 10% my data is not accurate.

Experiment: Carbonate Analysis – Abstract The purpose of this experiment was to find the percent of calcium carbonate in an unknown heterogeneous mixture and to determine the molar volume of carbon dioxide gas. To do this we needed to find the mass, volume, and moles of carbon dioxide. We collected this information with the use of a carbon dioxide apparatus and generator. The apparatus was constructed with a container of water, a graduated cylinder held in place, and a gas inlet. The carbon dioxide generator was composed of HCl, our unknown sample, and two test tubes. Once we performed the reaction we determined the volume of carbon dioxide evolved, the temperature of the bath, the barometric pressure, and the difference in mass of the sample. With this data, we are able to find our percent of calcium carbonate and the molar volume of the carbon dioxide at conditions of standard temperature and pressure. STP is when a single mole of an ideal gas occupies 22.4L. The major results in my experiment was my average volume of carbon dioxide at STP conditions which was 19.82 mol/L and the average percent of CaCO3 which was 86.71%. My standard deviation was 2.64, my relative standard deviation was 13.32%, and my %error was 11.52%. Since my data was above 10% my data is not accurate, it is considered that data under 10% is accurate and anything above arises questioning. Experiment: Carbonate Analysis - Introduction The goal of this experiment was to determine the percent calcium carbonate in a heterogeneous mixture and to determine the molar volume of carbon dioxide gas at 273 K and 760 Torr. A heterogeneous mixture is a mixture that is non-uniform and composition and its components can be separated using different mechanisms. Calcium carbonate is one of the most prevalent inorganic compounds on Earth and is found in many different forms such as chalk and marble. Calcium carbonate reacts well with acids to produce CO2 gas, represented by the reaction: CaCO3(s) + 2H3O+(aq) àCa2+(aq) + 3 H2O(l) + CO2(g). Calcium carbonate is treated with hydrochloric acid and CO2 gas is collected. We measure and calculate the CO2 generated in terms of mass, moles, and volume. We also collect the molar volume of carbon dioxide at STP. STP, stands for standard temperature and pressure, this is where one mole of an ideal gas takes up 22.4L. This volume of 22.4L is the standard volume for STP. An ideal gas is a hypothetical gas, in which the molecules that compose it have no intermolecular forces acting upon them beyond those of the collisions that occur between molecules and the collisions with the walls of their container. These molecules also do not take up space. We know that carbon dioxide is not an ideal gas, it is a real gas, which means it will not behave the same way as an ideal gas would. We will only slightly take this in to consideration when performing our calculations at STP but it shouldn’t have too much of an affect. This concept allows us to predict how real gases behave and correlates to the ideal gas law which is represented as PV=nRT, P represents pressure, V represents volume, T represents temperature, and R represents the gas constant which is 0.082 L•atm/K•mol. This represents pressure in atm but it could also be in the form of pascals. To find the molar volume of Carbon dioxide we need to find the mass of CO2(g) that evolves in a reaction so that this mass can then be converted to moles. We use STP conditions for both temperature and pressure. If we figure out the number of moles and volume at these conditions we can find the molar volume of CO2(g). We collect the CO2 in a reaction by displacing the water at an equal volume. Carbon dioxide that is collected is bubbled within the water, this CO2 is called “wet” CO2, this is because the volume is shared with water vapor. The pressure caused by the carbon dioxide gas and the water vapor is combined and known as the total pressure. CO2

is soluble in water, when it is being bubbled through water not all of the CO2 is accounted for because not all of it is measured as gas. Some of the CO2 gas actually dissolves in the water and is never accounted for, which is why we saturate the water with CO2 beforehand. Dalton’s law of partial pressures allows us to calculate the pressure of the dry CO2. Equations: (VCO2(STP)/nCO2) = molar volume of CO2 Dalton’s law of partial pressures = pCO2, expt = PT – pH2O Boyle’s and Charles’ law correction = VCO2( at STP) = VCO2, expt • (pCO2 expt (torr)/ 760 torr) • (273K/ TCO2 expt (K)) Percent in a mixture = (mass of CaCO3/ mass of mixture) • 100 = %CaCO3 Methods In this experiment, we needed to first fill a 1-L beaker with tap water and saturate it with CO2 by adding an Alka-Seltzer to the water. We want to make sure that the Alka-Seltzer is completely dissolved so that there is a homogenous solution of water and CO2. Instead of using a 1-L beaker we used a mini ice chest and filled it with water a little more than 2/3 of the way full. We want the graduated cylinder to be completely submerged in the water. We then collected the mass of the sample by calculating the amount of CaCO3 that would make 40ml of CO2 at STP. We collected this mass and transferred it into a 75-mm tube. We then set up the generator by adding HCl in a 200-mm test tube and then slowly added the 75-mm tube of CaCO3 into the 200-mm test tube and weighed it. We had to make sure that we did not spill or mix any of our sample, this would initiate a reaction and create a loss of CO2 gas. Once the generator was prepared we set up we prepared the CO2 collection apparatus by taking our cooler filled with CO2 and water and dipped a graduated cylinder inside to fill it with CO2 and water. To fill the graduated cylinder completely we place it horizontally in the water bath and tilt it just enough to that water can flow through. Once the graduated cylinder is completely filled with water you flip it upside down so that the mouth of the cylinder does not reach the surface and then the cylinder should be completely full of water. It is best to fill the graduated cylinder all the way but having air gaps is okay, if this happens it is important to take the initial reading to find the correct volume of water displaced for the calculations. Once the graduated cylinder is set up and secured in place, a gas inlet tube is inserted to connect the CO2 collection apparatus and the CO2 generator. It is important to make sure that the rubber stopper is secure on the CO2 generator and has not cracks or gaps. If there are gaps or cracks it should be switched out. This is because CO2 can escape, which would result in an inaccurate reading of less carbon dioxide than there actually was. The tube is set up at a 45° angle on a ring stand and secured. Once this is all set up you can initiate the reaction by gently moving the generator back and forth without letting the HCl run through the tubing, we only want to collect the gas released from the reaction. Once the reaction is complete and the CO2 is no longer evolved then the volume of CO2 is measured. In order to get an accurate reading, we have to submerge the graduated cylinder so that the water levels inside and outside the graduated cylinder reach equilibrium. We want them to be at equilibrium in order to get an accurate measurement of the amount of gas produced. We then collected the temperature, the barometric pressure, and the vapor pressure of water to calculate the dry CO2 collected from the reaction. The mass loss is also calculated by calculating the difference between the initial and final masses. The procedures for these experiments were written by

Beran, J.A. Laboratory Manual for the Principles of General Chemistry. (10th ed.); John Wiley & Sons, Inc. USA, 2015; pp 214-223. Data Table 1 Mass of sample (g) CO2 collected (ml) Pressure of dry CO2 (torr) Temperature (C°) H2O vapor pressure (torr)

Trial 1 0.2027 39.1 719.31 23.1 21.1

Trial 2 0.2013 39.8 719.31 23.1 21.1

In table 1 for the data we find the mass of the sample in grams by taking our initial mass of the generator and sample before the reactions and subtract it by the mass of the generator and sample after the reaction to find the total mass of the sample. To find the amount of CO2 in ml, we take the initial reading of our CO2 collecting graduated cylinder and subtract it by the final reading of the graduated cylinder. If the entire cylinder is filled with water, then the initial reading should be 0 and the total ml will be whatever amount of water was displaced by the gas. To find the pressure of the dry CO2, we must first have the barometric pressure of the room. The barometric pressure in the lab was 29.15 Hg. In order to convert this to torr we must multiply it by (25.40mmHg/1hg) to get 740.41 mmHg. Since mmHg = torr, out answer is 740.41. Then we find the vapor pressure of our water by looking up the pressure that correlates with the temperature of the water bath online. My water bath was 23.1°C, therefore my pressure was 21.1 torr. Then to find the pressure of the dry CO2 we need to subtract the 740.41 torr found from the barometric pressure by the 21.1 torr which is from the pressure of the water at a certain temperature to get the pressure of the dry CO2. To find the temperature of the water bath we simply recorded the temperature by placing a thermometer in the water. To find the vapor pressure of the water we just find the temperature of the water by using a thermometer and then look up the vapor pressure of the water at that specific temperature online. Results Table 1in the results displays the data for CO2 information. We need to find the mass of the CO2 evolved to find the moles. Once we find the moles by using molar mass we need we use this as part of finding the molar volume at STP. This is found using the units atm, and the temperature in K, specifically 273K at STP. We were then required to find our Standard deviation, rel. standard deviation, and the %error. The percent error was found by using 22.4 as the actual in the equation, this is the volume in liters that is used for STP conditions. This data shows that my experiment was not accurate, especially because I have a percent error above 10%. Table 1 Mass of generator and sample after reaction (g)

Trial 1 59.9523

Trial 2 59.8783

Mass loss of generator = mass CO2 evolved (g) Moles of CO2 evolved (mol) Pressure of dry CO2 (atm) Volume of CO2 at STP (L/mol) Molar volume of CO2 at STP (L/mol) Average molar volume of CO2 at STP (L/mol) Standard deviation Relative standard deviation (%) %error

0.0834

0.0726

0.0019 0.9465 0.4374

0.0016 0.9465 0.4452

17.95

21.69

19.82

19.82

2.64 13.32

2.64 13.32

11.52

11.52

Table 2 in the results displays the data regarding CaCO3. By finding the moles of CO2 we are able to find the moles of CaCO3 which then allows us to find the mass of CaCO3 in the sample by using molar mass to find this. We then found the percent mass which is found using the mass of the CaCO3 and the mass of the sample, with this we found the average of the two trials. Table 2 Moles CaCO3 in sample from mol CO2 generated (mol) Mass of CaCO3 in sample (g) Mass of original sample (g) Percent of CaCO3 in sample (%) Average percent of CaCO3 in sample (%)

Trial 1 0.0019

Trial 2 0.0016

0.1902

0.1602

0.2027 93.83

0.2130 79.58

86.71

86.71

Discussion In this experiment, we were expected to determine the percent of calcium carbonate in a heterozygous mixture and to determine the molar volume of carbon dioxide gas at STP conditions. The conditions we are using to find the molar volume are temperature at 273K and pressure at 760 torr. STP, stands for standard temperature and pressure, this is used when determining ideal gases. At STP, one mole of an ideal gas takes up a volume of 22.4L. An ideal gas is a hypothetical gas, one in which the molecules take up no space, have no interactions, and obey the laws applied to gases exactly. An ideal gas is also known as a theoretical gas, which means that the gas is composed of many particles moving randomly. The only interactions these particles have are elastic collisions. In terms of my results the average molar volume of carbon dioxide gas in my sample at STP was 19.82 L/mol. This was found by taking the volume of carbon dioxide gas at STP, and dividing it

by the number of moles of CO2 produced. I then took the average of the two and got my average molar volume. My standard deviation was 2.64, anything above a 1 indicates that there was high variation in my data. This high standard deviation indicates that my technique was not precise. As for my relative standard deviation I had a 13.32%, because this percent is higher than 10%, this is also an indication of my lack of precision in this experiment. Same goes for my present error, my 11.52% error is higher than 10% indicating that my experiment not only lacked precision but accuracy as well. Precision deals with how close two or more data points are to one another, while accuracy deals with how close a measured data set is to a set standard. In terms of the results I had for my experiment I lacked both precision and accuracy. I attempted to reduce the amount of error I had by re-doing my first trial. I had agitated the CO2 generator incorrectly and poured out all of my HCl. Then I re-did my trial and I believe that some of my CaCO3 got stuck on different parts of my tube and possibly didn’t react causing this error. When performing my second trial I realized that I still had some water in my tube that I did not dry. This could have also affected the masses and thus altering my results as well. The amount of water was very little but perhaps this could have had an effect on my data. I also had an issue with the scales, my masses would fluctuate and I would repeatedly have to tare the scale and reweigh. Having incorrect mass measurements could have also affected my results as well. In terms of other errors, I’m not quite sure as to why my error was so high, I had originally thought the experiment went quite well besides the first trial. Conclusion In conclusion, the average molar volume of CO2 in my sample at STP was 19.82 L/mol and the average percent of CaCO3 in my sample was 86.71%. My standard deviation was 2.64, my relative standard deviation was 13.32%, and my %error was 11.52%. Since my data was above 10% my data is not accurate. Errors Errors that occurred throughout my experiment was that I didn’t understand how to agitate the CO2 generator for my first trial so all of my HCl ended up in the CO2 collection apparatus, therefore I had to re-do my first trial. This didn’t give me an accurate measurement of the CO2 gas collected. A second error that occurred was that some of the CaCO3 didn’t react with the HCl because it would get stuck on the edges of the tube around the stopper when agitating. This would also give an inaccurate reading in terms of the amount of gas produced by the reaction. Errors that could have been that I forgot to add my Alka-Seltzer, therefore there wouldn’t be enough initial CO2 to begin with and a lot of the gas would dissolve in the water instead of being released as a gas. A second error could have been that I did not set up the CO2 generator correctly and some of the HCl and CaCO3 initiated a reaction before inserting the gas inlet. This would then cause there to be less CO2 accounted for than was actually produced. Experiment: Carbonate Analysis–Post Lab Questions 1. Part A.1. The water for the pan (Part A.3) is not saturated with CO2. Will the reported percent CaCO3 in the original sample be too high, too low, or unaffected? Explain.! If the water in the pan is not saturated with carbon dioxide then it will absorb some of the carbon dioxide. If this occurs then it will appear as if less carbon dioxide evolved. If it

2.

3.

4.

5.

appears that less CO2 is evolved, then it will also make it seem as if there was a lower percentage of CaCO3 in the original sample that was actually there.! Part A.2. Supposed 6M HCl is substituted for the 3 M HCl in the procedure. What would the consequence of this substitution? Explain.! If the 6M HCl was added instead of the 3M HCl then there will be more moles of carbonate present. If more moles are present this will not really have any affect the amount of CO2 produced. If the amount of HCl is higher than the amount of carbonate then the reaction will still undergo completion.! Part A.3. A few drops of HCl(aq) spilled over into the CaCO3 sample prior to firmly seating the stopper and prior to collecting any CO2(g). As a result of this poor technique, will the reported percent CaCO3 in the sample be too high, too low, or unaffected? Explain.! If some of the HCl spills over due to poor technique then this will affect the amount of CaCO3 reported. This is due to the fact that the reaction is put in effect and CO2 is being lost and not being collected. This would give us an inaccurate measurement of CO2 and as a result this will affect the percent of CaCO3, it will be too low.! Part A.4. The rubber stopper has a small crack resulting in a not-tight seal. How will this affect the calculated molar volume of CO2… too high, too low, or unaffected? Explain. ! If the rubber stopper has a small crack resulting in a non-tight seal then the molar mass of the CO2 will be lower than it should be. If there is a crack then CO2 might flow out of the crack. The decrease in the mass of the CO2 will then cause there to be a lower molar mass as well.! Part C.1. The water level in the CO2(g)-collection cylinder is higher than the water level outside the cylinder. See margin drawing. A) Is the wet CO2 gas pressure greater or less than atmospheric pressure? Explain. B) An adjustment is made to equilibrate the water levels. Will the volume of the wet CO2 gas increase or decrease? Explain. C) The student chemist chooses not to equilibrate the inside and outside water levels. Will the reported number of moles of CO2 generated in the reaction be too high, too low, or unaffected by this carelessness? Explain.! A. The wet CO2 gas pressure will be less than the atmospheric pressure, this caused by a higher water content which results in a lower atmospheric pressure. So if the water level in the graduated cylinder is higher than the water level outside then the CO2 pressure inside the cylinder will be less than the atmospheric pressure outside the graduated cylinder.! B. If an adjustment is made to equilibrate the water levels then the volume of the wet gas will decrease so that it is at equilibrium with the water levels. This event of equilibrium will happen because the wet gas pressure will be less than the atmospheric pressure on the outside of the graduated cylinder. If we want to increase the wet gas pressure then we need to decrease the amount of the wet CO2 gas.! C. If the student chemist does not equilibrate the inside and outside water levels, then the reported number of moles of CO2 for the reaction will be reported as too low. The pressure will be lower than it should be because the water level is higher in the graduated cylinder. The lower the pressure, the lower the number of moles recorded for the CO2 generated.!

6. Part C.1. An air bubble accidentally enters the CO2-collection graduated cylinder after the completion of the reaction. How does this error affect the reported moles of CO2(g) collected- too high, too low, or unaffected? Explain.! If an air bubble accidentally enters the graduated cylinder after the reaction is completed then t...