Experiment. Molar Mass of an Unknown Sulfate Salt by Gravimetric Techniques 1 This lab is to reacquaint you with some basic laboratory techniques and serves as a warm-up to the experiments in this course. Lab Overview: The amount of sulfate ions in a product will be determined by precipitation of its salt as a secondary BaSO 4 product. Stoichiometry calculations will be used to identity of the molar mass of the sulfate salt. A strategy to obtain the best results, is to isolate BaSO 4 as large crystals. Two methods will be used to dry the crystals, one is by using glass frit funnel filtration technique and the second is to collect the precipitate using ashless filter paper and then igniting the precipitate to recover the BaSO4. The two results will be compared to determine if one method is more effective over the other. 1. Read Chapter 27 in Harris (9 th Ed) or Chapter 26 (8 th Ed), before beginning this experiment. Reagents and Equipment: The lab techs will prepare the following solution for the class- Crucible and Cover Ashless filter paper (110 mm diameter) 0.1 M BaCl 2 solution Glass frit funnel Alkali or alkaline salt unknown 6 M HCl solution 25-mL Vol Pipet Safety: Be careful when handling 6 M HCl. If any HCl comes in contact with your skin or eyes you should immediately wash with water for several minutes. You should be wearing your lab coat, gloves and your goggles at all times. Make sure to allow ample time for the crucible to cool after it has been heated. Gravimetric analysis is a quantitative method for accurately determining the amount of a substance by selective precipitation of the substance from an aqueous solution. The precipitate is separated from the remaining aqueous solution by filtration and is then weighed. Assuming that the chemical formula for the precipitate is known and that the precipitation reaction is stoichiometric (goes to completion) the mass of the substance in the original sample can be determined. In this experiment, you will determine the percentage (by mass) of sulfate in an unknown sulfate salt by gravimetric analysis. First you will dissolve a measured mass of the unknown salt in water. ext you will add an excess of aqueous barium chloride to the aqueous solution of the unknown salt. This will result in the precipitation of the sulfate as barium sulfate. BaCl 2 (aq) + M 2 SO 4 (aq) _BaSO 4 (s) + 2 MCl (aq) (assuming +1 cation) BaCl 2 (aq) + MSO 4 (aq) _BaSO 4 (s) + MCl 2 (aq) (assuming +2 cation) The number of moles of sulfate can be determined from the mass of the barium sulfate. Since barium chloride is added in excess, and since the precipitation reaction is assumed to go to completion, the number of moles of sulfate recovered in the precipitate can be assumed to be equal to the number of moles of sulfate in the original sample allowing for the calculation of the percentage by mass of sulfate in the original sample. For best results the BaSO 4 crystals should be as large as possible. This facilitates filtration and washing of the crystals and the decreased surface area minimizes the amount of impurities adsorbed onto the crystals. Generally, the largest crystals are obtained when the rate of precipitation is as low as possible. The rate of precipitation is minimized by slowly adding the BaCl 2 solution to the aqueous mixture containing the unknown salt while continuously stirring the mixture. The rate of precipitation can be slowed down still farther by slightly increasing the solubility of the BaSO 4 (remember that a substance that is said to be insoluble is in fact very slightly soluble). The increase in solubility is achieved by lowering the ph with 6M HCl and by increasing the temperature. It has been shown that the resulting decrease in the yield of the BaSO 4 is insignificant under these conditions.
Procedure Work with another partner with one using the sintered-glass funnel technique and the other using ashless paper combustion procedure. Each will work on the same unknown and work independent of each other except to report results. Use deionized water in all the procedures describe here. Use chilled deionized water when washing the BaSO 4 precipitate in step 5 below. At a minimum, you and your partner should do three trials each. 1. Measure and record (nearest 0.0001 g) the mass of a clean, dry 250-mL beaker using the analytical balance. Add between 0.30 g and 0.35 g of your unknown sample to the 250-mL beaker and record (nearest 0.0001 g) the mass of the beaker plus sample. 2. Add 50 ml deionized water to the sample in the beaker. ext add 20 drops of 6 M HCl to the beaker. Stir the contents of the beaker until the sample has entirely dissolved. Leave the stirring rod in the beaker. 3. Measure 25 ml of 0.1xx M BaCl 2 solution using a volumetric pipet to a 50-mL Beaker. The beaker should be clean and rinsed with deionized water but need not be dry. 4. Heat the solution containing the sample in the 250-mL beaker until it is nearly (but not quite) boiling. Turn the burner off and slowly pour small portions of the BaCl 2 solution into the 250-mL beaker containing the sample. This step should take at least 3 minutes otherwise the BaCl 2 is being added to rapidly. Stir the contents of the beaker as you add the BaCl 2 solution. You should observe the formation of the white BaSO 4 precipitate. Be sure to use all the BaCl 2 solution since the unreacted barium ions will be analyze in experiment 2. Use the wash bottle to rinse the BaCl 2 solution in the 50-mL beaker and pour this solution in to the 250-mL beaker containing the barium sulfate precipitate. Try to use a minimum amount of water in this step however. 5. Rinse any precipitate that remains on the stirring rod with cold deionized water into the solution with a small amount of deionized water. Allow the precipitate to settle in the beaker in a cold-water bath for about 20 minutes. a) Procedure for ashless filter paper isolation: 6a. For the lab partner using the ashless paper procedure, prepare your crucible as follows: While the precipitate settles prepare your crucible by heating it (with the cover on) in the hottest part of the Bunsen burner flame for about 2 minutes. After the crucible has cooled to room temperature use the analytical balance to record the mass of the crucible and the cover to the nearest 0.1 mg or 0.0001 g. Repeat the procedure until successive weighing agrees to within 0.5 mg (0.0005 g). If you cannot get with in 0.5 mg after four cycles, then proceed if you are within 1-mg. You will be deducted a few points for your technique but at least you can proceed on with the experiment. The number of significant figures of your mass should be consistent with the precision of the balance you are using. Write in your data table the difference in mass between consecutive weighing. 7a Set up the gravimetric filtration by obtaining a piece of ashless filter paper and fold it into quarters. Open the folded paper into a cone, place it into a funnel and wet the filter paper with cold deionized water so that it adheres to the funnel. Place a clean 500-ml Erlenmeyer flask under the funnel to collect the filtrate. 8a After 20 minutes has passed slowly pour the mixture containing the BaSO 4 precipitate down your stirring rod into the funnel. Be careful that the level of liquid in the funnel is never more than three-fourths of the way to the top of the filter paper. When the transfer is complete use your wash bottle (filled with chilled deionized water) to rinse the residual precipitate from the beaker and the stirring rod into the funnel. 9a. Take care to collect all the filtrate in each of your filtration process. The filtrate contains the unreacted excess barium ions that didn t form BaSO 4. The barium will be analyzed via EDTA titration in the next experiment. 10a After all the liquid has drained from the funnel very carefully press the top edges of the filter paper together, and fold the filter paper into a compact package that will fit into the crucible. In order to avoid tearing the filter paper it is important that you do not use too much force. Place the folded filter paper into the crucible. 11a Set up your ring stand in the fume hood and support the crucible in a clay triangle that is attached to your ring stand. Gently heat the crucible without the cover to remove the water. After several minutes when you are sure the paper is dry, heat the crucible more vigorously so that the filter paper begins to char (turning from white, to brown, to black) but not so vigorously that the filter paper bursts into flame. If the filter paper bursts into flame you should smother it with your crucible cover and lessen the amount of heat. Continue to heat moderately until all of the filter paper has turned black. 12a Once all the filter paper has turned black heat the crucible vigorously with the cover off in the hottest part of the Bunsen burner flame so that the bottom of the crucible is red hot. The charred filter paper (carbon) will gradually combust and be converted into CO 2 gas. When the filter paper is entirely combusted only the white BaSO 4 should remain in the crucible. The crucible should be heated vigorously until there is no charred filter paper remaining. 13a Allow the crucible to cool. When the crucible has cooled to room temperature, record the mass of the crucible, the cover and its contents to the nearest 0.001g on the analytical balance. 14a Store the product in the crucible, the BaSO 4 precipitate in another container. The barium content can also be analyzed in a future experiment using the ICP-AAS instrument.
b) Procedure for sintered-glass funnels isolation: 6b. For the lab partner using the sintered-glass funnels prepare your funnels as follow: While the precipitate settles, clean a three medium-porosity, sintered-glass funnels. Prepare about 50 ml of 1:1 HCl by adding 25 ml of concentrated (12M) HCl to 25 ml of deionized water. (ALWAYS ADD ACID TO WATER). Rinse the medium-porosity, sintered-glass funnel with a total of 10-15 ml of 1:1 HCl, drawing small volume of the acid through the funnel by suction. Discard the HCl rinses in the collection flask to the appropriate waste container. Rinse the funnel at least three times with deionized water, drawing a reasonable amount of deionized water through the funnel. Slowly break the suction of the filtering flask. Discard the water rinses in the filter flask. ext, wash each funnel one at a time, with acetone from an acetone-washing bottle. Rinse the funnel several times with a stream of acetone and draw air through the funnel for about five minutes. 7b. Dry the sintered-glass funnels by placing in a large enough beaker to hold the funnels and then placing in an oven to dry for 1 2 h at 105 C in an oven. Cool the sintered-glass funnels in a desiccator for 30 min. and weigh. Repeat the procedure with 30-min heating periods until successive mass readings agrees to within 0.3-0.5 mg (0.0005 g). Use a paper towel or tongs, not your fingers, to handle the funnels. Alternatively, a 900-W kitchen microwave oven dries the sintered-glass funnels to constant mass in two heating periods of 4 min and 2 min (with 15 min allowed for cool down after each cycle). You will need to experiment with your oven to find appropriate heating times. The number of significant figures of your mass should be consistent with the precision of the balance you are using. Your data table should record the difference in mass between consecutive weighing. 8b Set up for suction filtration using two side-arm Erlenmeyer flasks, see figure. Be sure the primary Erlenmeyer flask is clean of any residue since the filtrate will need to be collected. Take care to collect all the filtrate in each of your filtration process. The filtrate contains the unreacted excess barium ions that didn t form BaSO 4. The barium will be analyzed via EDTA titration in the next experiment. 9b Begin by filtering each solution through these weighed sintered-glass funnels using vacuum filtration. ext add ~3 ml of ice-cold water to the beaker, and use a rubber policeman to help transfer the remaining solid to the funnel. Repeat this procedure with small portions of ice-cold water until all of the precipitate has been transferred to the funnel. Finally, use two 10-mL portions of ice-cold water to rinse each beaker, and pour the washings over the precipitate. Save each of the filtrate in your three trials in different containers. 10b Dry the precipitate by aspirator suction for 1 min, then in an oven at 105 C for 1 2 h. Bring each filter to constant mass. The product is somewhat hygroscopic, so only one filter at a time should be removed from the desiccator. Weighing should be done as quickly as possible. Alternatively, the precipitate can be dried in a microwave oven once for 4 min, followed by several 2-min periods, with cooling for 15 min before weighing. 11b Upon completion of the experiment, discard the solid precipitate in a chemical clean container. The barium content can also be analyzed in a future experiment. 12b Wash out the sintered glass funnels with 1:1 HCl solution as done at the beginning of this procedure, 6b. For more information on technique used in the lab go to the following links: http://zimmer.csufresno.edu/~davidz/chem105/dandw/dryw2.html http://abacus.bates.edu/~ganderso/biology/resources/serological_pipet.html http://www.youtube.com/watch?v=otye_rfarlc http://www.youtube.com/watch?v=x7l4el9nrs&feature=relmfu https://youtu.be/d3ekbt8fg9a
Calculations: Moles of Analysis A. Moles of sulfate in BaSO 4 Moles SO 4 2- = mass ppt (g) 1 mol BaSO 4 g BaSO!## "## $ 4 2-1 mol SO 4 1 mol BaSO 4 molar mass BaSO 4 B. Molar mass of unknown sulfate salt: C. Identity of metal in unknown sulfate salt. Mass of cation metal in sulfate salt = molar mass sulfate salt - molar mass sulfate = atomic weight of metal (g/mol) *if the metal is alkali, then divide atomic weight of metal by 2. D. Report the standard deviation, the relative standard deviation (% RSD) or the coefficient of variation (CV) (Use Excel) See below see calculations for these statistical pamameters. ext carry out a Comparison of Means with Student s t. Decide which case (page 72, Harris, 9 th edition) you should use to compare your results with your partner Carry out the analysis and report your results as shown in the sample data page. Statistical Analysis: i) Standard Deviation: For a population, the measure of the absolute precision is the standard deviation. This is found by the equation: σ = (x i µ) 2 Eq1 In the real world, it is rare to have a large number of measured value to work with. It is much more common to have 3-5 values. Moreover, for unknown samples the true values is not known. While the expression in equation 1 (s) is the formal definition of standard deviation, it is easier to calculate the standard deviation from the following formula: s = (x i x) 2 1 = 2 d i 1 Eq2 ii) Variance: Another way of measuring precision is the variance. The variance is the standard deviation squared (s 2 ). s 2 = (x i x) 2 d i 2 = 1 1 Eq3 iii) Relative standard deviation, coefficient of variation and confidence interval: Relative Standard Deviation (RSD): Standard deviation in relative terms is defined as standard deviation divided by the mean. The relative standard deviation can also be expressed in parts per hundred (%) by multiplying RSD by 100, this is also the coefficient of variation, CV. Confidence interval (90, 95 and 99%): RSD = s x Eq4 % RSD = CV s *100, pph x pph = parts per hundreds or % Eq5 Confidence interval = x + t s n
Test for Outlier Apply a Grubb s Test and Q-Test for any suspected outliers at 95 % level. See page 83 of text for critical values for 95% confidence. If your results show an anomalous data then use the Q-test to determine if the result should be rejected. Q = (Suspected Value - earest Value) (Suspected Value - Furthest Value) Eq7 Questionable value - x G calc = s Eq8 Table of Data, Results and Statistical Analysis: (Data and results for both partner should be reported) Unknown Raw Data 1. Unknown number= 2 Mass unknown (Each trial) 3 Mass of crucible (or funnel) and tolerance of final weighing. 4 Final Mass of crucible or funnel with precipitate & tolerance. 5 Mass of precipitate, each trial 6 Moles of sulfate, each trial Analysis and Results 7 Molar mass of sulfate salt (Average) 8 Mass of cation in sulfate salt (Average) 9 Identity of cation in sulfate salt Statistical Analysis 10 Averages and Standard deviations of 7 and 8 11 Variance, RSD and CV for 7 and 8 12 Comparison of Mean with Student s t (Selected case) 13 90%, 95% and 99 % confidence level 14 Q & G Test (95%) for any outlier. Discussion The goal of this experiment was to determine the identity of an unknown metal sulfate salt and to apply statistical analysis on the results. Write an appropriate discussion for this experiment. Some things to consider in your discussion are: Is the insolubility of the barium sulfate salt relative to the unknown metal salt critical for this experiment to succeed? Why? Why was excess BaCl 2 is added to the unknown solution. Why is it important to repeatedly weight the crucible and the sintered glass-funnel to constant weight? Consider the possible scenario. Consider if one of the empty sintered glass-funnel never makes tolerance (0.3 mg) with the other two funnels making tolerance. In the experiment, there was success in the weighing of the BaSO 4 product in two of the three trials (the sintered glass-funnels that met tolerance). The experiment is not complete, however, until a third result is recorded. How can this experiment be modified so the third precipitate can be weighed without using a repeating the entire experiment from start? 1 C. H. Hendrickson and P. R. Robinson, J. Chem. Ed. 1979, 56, 341. 2 R. Q. Thompson and M. Ghadiali, J. Chem. Ed. 1993, 70, 170. Sample Data Table Sample Unknown # 1 2 3 4 Mass of crucible/funnel (last weighing), (g) Mass of ppt in crucible/funnel (last weighing), (g) (A) Mass of ppt, (g) (B) Moles of sulfate (moles) Average Standard Deviation Variance, RSD % RSD (CV) (C) molar mass of unknown sulfate salt (D) Atomic weight of cation in unknown sulfate salt (E) 90% confidence level (F) 95% confidence level (G) 99% confidence level Average Range +/- + - Questionable Value Q Calc, G Calc ew Avg. ew Range +/- (95 % CL) Q-Test Indicate outliner (Atomic weight unknown) G-Test Indicate outliner (Atomic weight unknown)
Gravimetric Determination of Molar mass of an unknown sulfate salt % of Score # CRITERIA (Tentative point distribution - may change depending on experiment) pts % Score 1 Prelab Questions 2 Introduction and Procedures A. Introduction Objective of Expt. Background information. Math relationship used in study. B. Procedures Outline of procedures in Expt. Flow chart pictorial of procedures. Procedural changes. Information (data) to be recorded during expt. (to be presented in Table form.) Safety and disposal information. This portion of the report should be turned in before the start of lab class (prelab discussion). 3 Data, Observe., Results and Calc. C. Data and Observation Data in table form. & detailed observation written in the table. All data entry should contain the proper number of significant figures and units. Data should always be recorded in an organize fashion. Balance chemical equations; all chemical reaction which occurred during an experiment should be written in this section. Then it should also be written in the discussion portion of the report. This portion of the report should be turned in before you leave the laboratory. Calculations & Results D. Calculations Sample calculation shown Statistical analysis of data and result (if applicable) E. Results Result(s) in table form. In this section accuracy of results is very important as well as detailed calculation showing how the result was obtain. "Unknown" will also be included in this section. 4 Discussion (Talking points) F. Discussion What is your final result in this experiment. Are the three trials consistent with each other? If not what would account for the inconsistencies? Were you able to get within 0.5mg in your weighting? If you had problems, how did you resolve this? G. Conclusion Summary of the goal of the experiment and how that goal was achieved in the experiment. H Post-lab questions or Editorial comment What did you learn in this experiment? What skills in lab practice did you develop through this expt? This portion (Calculation and Discussion) is turned in at the beginning of class of the due-date 5 Overall Presentation (of lab notebook) Lab technique during experiment; example are- class preparation, safety glasses precautions and leaving the laboratory clean. Report presentation: examples are the headings of each report that includes name, title, lab partner, date and section #. Legibility of report. Is the report easy to read or is important information jotted down by small print in the corners of the lab report? The overall impression is important. 6 Lab Technique Safety: wear goggles, handle chemicals with caution, proper handling of lab equipment Leave lab clean and tidy 10% 15% 30% 20% 15% 10% Unknown #: Results: % Error: Score /20 Total (This total may be adjusted depending on lab technique and student conduct in the experiment)
otes and Modification (Ashless procedure) 1. The sample is dissolved, if it is not already in solution. 2. The solution may be treated to adjust the ph (so that the proper precipitate is formed, or to suppress the formation of other precipitates). If it is known that species are present which interfere (by also forming precipitates under the same conditions as the analyte), the sample might require treatment with a different reagent to remove these interferents. 3. The precipitating reagent is added at a concentration that favors the formation of a "good" precipitate (see below). This may require low concentration, extensive heating (often described as "digestion"), or careful control of the ph. Digestion can help reduce the amount of coprecipitation. 4. After the precipitate has formed and been allowed to "digest", the solution is carefully filtered. The filter is chosen to trap the precipitate; smaller particles are more difficult to filter. o Depending on the procedure followed, the filter might be a piece of ashless filter paper in a fluted funnel, or a filter crucible. Filter paper is convenient because it does not typically require cleaning before use; however, filter paper can be chemically attacked by some solutions (such as concentrated acid or base), and may tear during the filtration of large volumes of solution. o The alternative is a crucible whose bottom is made of some porous material, such as sintered glass, porcelain or sometimes metal. These are chemically inert and mechanically stable, even at elevated temperatures. However, they must be carefully cleaned to minimize contamination or carryover(crosscontamination). Crucibles are often used with a mat of glass or asbestos fibers to trap small particles. o After the solution has been filtered, it should be tested to make sure that the analyte has been completely precipitated. This is easily done by adding a few drops of the precipitating reagent; if a precipitate is observed, the precipitation is incomplete. 5. After filtration, the precipitate including the filter paper or crucible is heated. This achieves three purposes: o The remaining moisture is removed (drying). o Secondly, in some experiments the precipitate is converted to a more chemically stable form. For instance, calcium ion might be precipitated using oxalate ion, to produce calcium oxalate (CaC 2 O 4 ); it might then be heated to convert it into the oxide (CaO). It is vital that the empirical formula of the weighed precipitate be known, and that the precipitate be pure; if two forms are present, the results will be inaccurate. o The precipitate cannot be weighed with the necessary accuracy in place on the filter paper; nor can the precipitate be completely removed from the filter paper in order to weigh it. The precipitate can be carefully heated in a crucible until the filter paper has burned away; this leaves only the precipitate. (As the name suggests, "ashless" paper is used so that the precipitate is not contaminated with ash.) 6. After the precipitate is allowed to cool (preferably in a desiccator to keep it from absorbing moisture), it is weighed (in the crucible). The mass of the crucible is subtracted from the combined mass, giving the mass of the precipitated analyte. Since the composition of the precipitate is known, it is simple to calculate the mass of analyte in the original sample.