Abstract lab techniques of weigh by difference, quantitative


AbstractThe goal of thisexperiment was to determine the amount of calcium in an unknown sample byconverting the calcium into CaC2O4H2O through gravimetric analysis.Ca2+(aq) + C2O42-(aq)+ H2O(l) ® CaC2O4×H2O(s)Three portions of0.4347g, 0.3982g and 0.3938g of the unknown sample were used. The mass of the calcium oxalatemonohydrate crystals formed were 0.

5022g, 0.4554g and 0.4551g. The percentageof calcium by mass in the three samples were 31.69%, 31.37% and 30.70%.

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Theaverage percentage of calcium by mass in the unknown sample was found to be31.58% with a standard deviation of 0.149 and a relative error of 2.27%. The labtechniques of weigh by difference, quantitative transfer, gravimetric analysisand homogeneous precipitation were used in this lab. IntroductionThe primary purpose ofthis lab is to determine the content of calcium in an impure sample of calciumcarbonate by converting the calcium into CaC2O4H2O through gravimetric analysis. Ca2+(aq) + C2O42-(aq)+ H2O(l) ® CaC2O4×H2O(s)The lab techniques ofweigh by difference, quantitative transfer, homogeneous precipitation andgravimetric analysis were used in this lab. Gravimetric analysis is the methodin which the percent composition of a substance can be calculated by analyzingthe masses of reactants and products.

For gravimetric analysis, an insolublesolid of the desired compound must form; the precipitate must be pure; and thisinsoluble compound must be easily weighed and handled. Due to the insolubilityand the ease of weighing of CaC2O4×H2O, it is chosen as the desiredproduct of the experiment. This allows the filtration process to be successful(as CaC2O4×H2O can exist as large crystals that are easy to be filtered)and so the final measurements will be accurate and precise. Homogeneousprecipitation is the steady and slow precipitation that ensures the formationof large solids instead of small crystals. This is done by heating thereactants at a relatively low temperature (near boiling point) which allows thereaction to occur very slowly.

The calcium ions in the unknown sample must beconverted slowly into calcium oxalates to ensure maximum yield and purity.Using urea as a weak base of ammonia which reacts slowly can successfully slowdown the formation of the CaC2O4×H2O crystals. The overall reactionsare listed as below:CaCO3(s) + HCl(aq) ® CaCl2(aq) + H2O(l) + CO2(g)CaCl2(aq) ® Ca2+(aq) + 2Cl-(aq)(NH4)2C2O4(aq) + HCl(aq)® HC2O4-(aq)+ 2NH4+(aq) + Cl-(aq)(NH2)2CO(s) + H2O(l)® CO2(g) + 2NH3(aq)NH3(aq) + H+(aq) ® NH4+(aq)HC2O4-(aq) ? C2O4-(aq)+ H+(aq)Ca2+(aq) + C2O4-(aq)+ H2O(l) ® CaC2O4×H2O(s) Excess 1M HCl need tobe added at an early stage of the experiment to ensure the pH is under 2. Asolution with a low pH will allow the successful formation of HC2O4-instead of C2O42-. C2O42-can immediately combine with Ca2+ and form small CaC2O4crystals which is not desired in the early stage of the experiment. Anotherreason for adding excess HCl is that a lot of HCl is required to make sure thatall of the calcium in the calcium carbonate solid are changed into free Ca2+.By doing this, the results of the experiment will be more accurate.

The pH ofthe solution must be increased later in the lab. This is because a highconcentration of C2O42-is needed to precipitate out all of the Ca2+ in the solution. HC2O4-(aq) ? C2O4-(aq)+ H+(aq)As the pH of the solution increases, there are more OH- ionsthan H+ ions in the solution. Based on Le Chatelier’s principle, thesystem would favor the forward reaction to produce more H+, thus moreC2O42- can be produced. Gravimetric analysis isan important method in the world of science as it is one of the most accuratemethod of macro quantitative analysis. For example, thermo-gravimetric analysiswas used in the field of biology to study the mass losses of biomass and thepercent of evolved species through biomass pyrolysis1.Thermo-gravimetric analysis was used to obtain data sets to calculate the yieldof liquids and yields of pyrolyzed gas and compare them1.

Gravimetricanalysis is also applied to a gas-liquid chromatography study2.Fatty acid methyl esters were streaked on to prepared chromatograph plates andexamined. Groups of saturated, tans-monounsaturated, cis-monounsaturated,di-unsaturated and poly-unsaturated esters formed and the proportion of thesegroups were determined gravimetrically, hence the first three groups can beidentified and separated through gas-liquid chromatography2.Gravimetric analysis was also used to study moisture diffusivity3.

The mass loss of a sample through a non-isothermal procedure was determinedusing thermo-gravimetric analysis to program a heating profile which werecompared to original isothermal procedures to interpret the temperaturedependence of the moisture diffusivity3. ExperimentalThe procedures of this lab were adaptedfrom the procedures in the chem 203 lab manual4. 0.3-0.5 g of theunknown were weighed out in each 3 labeled 250 mL beakers using the techniqueof weigh by difference.

100 mL of deionized water were added to each 3 samples.And 1M HCl were then added to the beakers until the pH reached 2. The amount ofHCl added were recordedto be 7.0 mL, 7.0 mL and 7.0mL respectively. 4 drops of Methyl red indicator were then added toeach of the solution.

3 portions of 20 mL of ammonium oxalate were measured outand poured into three 50 mL beakers. 3 portions of 1.0 mL of 1M HCl were thenadded to each of the ammonium oxalate in the beaker and the pH were checked andensured to be less than 2. The solutions were then added to the samples thatwere pH adjusted and contains the methyl red indicator. Enough solid urea wasthen added to the solution.

The amount of urea added were 11.1179g, 11.0474gand 11.2856g respectively. Swirling was applied to dissolve all the urea. The 3samples were then heated to near boiling point on a hot plate.

3.0045g, 3.0225gand 3.0024g of additional urea were added in the first half hour. As theindicator did not change color, more urea were added – 5.0190g, 5.

0116g and5.0121g. Then, 5.0484g, 5.0253g, 5.0223g and 5.

0560g, 5.0145g, 5.0139g ofadditional amount of urea were added to fully react. After the solution turnedfrom pink to yellow, the liquids on the watch glass and the stirring rod wererinsed into the beakers.

Vacuum filtrationapparatus was set up and the hot solutions were filtered through the sinteredglass crucibles to isolate the calcium oxalate monohydrate crystals. Smallamounts of deionized water were used to transfer all precipitations into thecrucible. After filtration, the crystals were washed using 2 portions of 15 mLice cold deionized water and rinsed with another 2 portions of 10 mL acetoneover the vacuum filtration apparatus. Air were drawn through the crystals todry the crystals and remove the traces of acetone. Then the samples were dry inair for another half hour.

Then the crucibles were put in a beaker and dried at105°C. After drying, the crucibles were cooleddown and weighed. After first weighing, the crucibles were put back into thedessicator for 15 minutes and reweighed. This was repeated until a constantweight was obtained.  ResultsThe mass of the unknownsamples that were measured using weigh by difference are 0.4347±0.0002g, 0.3982±0.

0002g and 0.3938±0.0002g(Table 1).Table 1. Mass of the unknown sample (g)   #1 #2 #3 Mass 0.

4347±0.0002g 0.3982±0.

0002g 0.3938±0.0002g  The 3sintered crucibles that used for vacuum filtration was measure to have massesof 31.4510±0.0002g, 31.4015±0.0002g and 32.8162±0.

0002g (Table 2).Table 2. Mass of Crucibles (g)   #1 #2 #3 Mass 31.4510±0.0002g 31.4015±0.0002g 32.

8162±0.0002g  The volumes of HCladded were recorded (Table3).Table 3.

Volumes of HCl added   #1 #2 #3 The amount of HCl added to dissolve the sample 7.0 mL 7.0 mL 7.0 mL The amount of HCl added to react with (NH4)2C2O4 1.0 mL 1.0 mL 1.

0 mL  Using the amount of the1 M HCl that were added to dissolved the sample, the percentage of the mass ofcalcium in the unknown sample can be predicted. Since 7 mL of HCl were added todissolved the sample, the number of moles of HCl can be calculated from itsconcentration and volume. n(HCl) = HCl  V(HCl) = 1 mol L-1  0.007 L = 0.007 molThis shows that the number of moles of CaCO3 is also 0.007mol from the balanced equation.

CaCO3(s) + HCl(aq) ® CaCl2(aq) + H2O(l) + CO2(g)n(HCl) = n(CaCO3) = 0.007 molTherefore, the mass ofCaCO3 and the percentage of calcium by mass in the sample can becalculated:m(CaCO3) = molar mass(CaCO3) n(CaCO3) = 100.09 gmol-1  0.007 mol = 0.70063 gm(Ca) = m(CaCO3)  = 0.

28056 gCa % =  = 64.54%Table 4. Prediction of Ca% based on the amount of HCl added.   n(HCl) m(CaCO3) m(Ca) Ca % #1 0.007 mol 0.70063 g 0.28056 g 64.54% #2 0.

007 mol 0.70063 g 0.28056 g 70.45% #3 0.007 mol 0.70063 g 0.

28056 g 71.23%  Average prediction of percentage of Ca by mass = 68.74% A lot of solid ureawere measured out and added to the solution to fully react, the amount of ureaadded to the solution are recorded below (Table 5).

Table 5. Mass of urea added   #1 #2 #3 Mass of urea added before the solution was heated 11.1179±0.0002g 11.0474±0.0002g 11.2856±0.0002g Mass of urea added while heating (1st time) 3.

0045±0.0002g 3.0225±0.

0002g 3.0024±0.0002g Mass of urea added while heating (2nd time) 5.0190±0.0002g 5.0116±0.0002g 5.

0121±0.0002g Mass of urea added while heating (3rd time) 5.0484±0.0002g 5.0253±0.

0002g 5.0223±0.0002g Mass of urea added while heating (4th time) 5.

0560±0.0002g 5.0145±0.

0002g 5.0139±0.0002g   Total amount of urea added 29.2458±0.0002g 29.1213±0.

0002g 29.3363±0.0002g  The mass of thecrucibles with the crystals after heated in the oven and reweighed every 15minutes after cooling down are recorded below (Table 6).Table 6. Mass of crucibles after heating   #1 #2 #3 Mass of crucibles after heating 31.9491±0.

0002g 31.8537±0.0002g 33.2683±0.0002g Mass of crucibles reweighed after 15 mins 31.9529±0.

0002g 31.8565±0.0002g 33.2708±0.0002g Mass of crucibles reweighed after 20 mins 31.9532±0.

0002g 31.8569±0.0002g 33.2713±0.0002g  To calculate thepercentage by mass Ca in the unknown sample, the mass of the calcium oxalatemonohydrate that produced during the lab need to be determined. This is done byfinding the difference between the mass of the crucible containing the crystalsand the mass of the sintered crucibles.Sample calculation: m(CaC2O4H2O) = m(crucibles with CaC2O4H2O) – m(crucibles)= 31.

9532 g – 31.4510 g= 0.5022 gThen, the number ofmoles of Ca can be determined using the mass and the molar mass of CaC2O4H2O. n(Ca) = n(CaC2O4H2O) =  =  = 3.43710-3 molThe mass of Ca in thesample can then be determined.m(Ca) = n(Ca) molar mass(Ca) = 3.43710-3 mol  40.

08 g mol-1 = 0.1378gAnd the percentage bymass of Ca in the unknown sample can be calculated (Table 7).%Ca =  100 =   100 = 31.69%Table 7. Percentage by mass of Ca   Mass of CaC2O4H2O Moles of Ca Mass of Ca Ca percentage by mass #1 0.

5022 g 3.43710-3 mol 0.1377 g 31.67% #2 0.

4554 g 3.11710-3 mol 0.1249 g 31.37% #3 0.4551 g 3.11510-3 mol 0.1248 g 31.

70%  And the averagepercentage by mass of Ca in the unknown sample is  = 31.58%.To evaluate the qualityof this data, a Q-test must be taken. To carry out the Q-test, the differencebetween the questionable value and the nearest value in the data set isdetermined and divided by the range of the set.for crucible #2,   0.909The Qepx isthen compared to a value listed on a Q-test table corresponding to the numberof values in the data set and the degree of confidence desired in the rejectionprocess. For a data set of 3 observations, the Qcrit is 0.940.

The Qepxfor each trial are calculated below in Table 8.Table 8. Qepx for each trial and comparison of Qepx withQcrit   #1 #2 #3 Qepx 0.091 0.909 0.091  Since all of the Qepxvalues are under 0.940, none of them should be rejected with a 90%confidence level.

Thus, the mean percentage by mass of Ca in the unknown is31.58%.The actual percentageby mass of Ca in the unknown sample is 30.88%, so the relative error andstandard deviation of the data collected must be calculated.

  = = 0.149 DiscussionThe determination ofthe mass percentage of calcium in an impure sample of CaCO3 was thegoal of this lab. This is done by using the method of gravimetric analysis.Gravimetric analysis is the technique in which the percent composition of asubstance can be calculated by analyzing the masses of reactants and products.The techniques of quantitative transfer, weigh by difference, homogeneousprecipitation and vacuum filtration were also applied to this lab.

Quantitativetransfer is important for this lab, it ensured all the precipitates to berinsed and collected. The technique of quantitative transfer and weigh bydifference allowed the data collection to be very accurate and precise. Sincethere are 3 special requirements for gravimetric analysis, calcium oxalatemonohydrate was chosen as the desired product.

The requirements are: 1. Thecompound formed must be an insoluble solid; 2. This solid precipitate must bepure; 3. The insoluble compound must be easy to be weighed. Calcium oxalatemonohydrate was chosen because it exists as large crystals with a very low solubility,so filtration can be successful (all the calcium can be collected) and the datacan be accurate. Homogeneous precipitation is the steady and slow precipitationto ensure the formation of large crystals instead of small crystals.

This isdone by heating the reactants at a relatively low temperature (near boilingpoint) which allows the reaction to occur very slowly.  The pH played a crucialrole in this lab. The pH was checked to be less or equal to 2 to ensure theformation of HC2O4-, otherwise crystals of CaC2O4would form right after the (NH4)2C2O4were added. Additionally, a low pH means there was an excess of HCl in thesolution. This is favored because an excess amount of HCl is needed to dissolveall the CaCO3 and change all of the calcium into free calcium ions.

Inthe second part of the experiment the pH of the solution was increased to above6. This is because a high concentration of C2O42-were needed to fully react with all of the Ca2+. If the pH of thesolution is increased, according to Le Chatelier’s principle, the system wouldreact to produce more H+ to reverse the change.

Therefore, thesystem favors the forward reaction and so more C2O42-can be produced.HC2O4-(aq) ? C2O42-(aq)+ H+(aq)Also, to ensure maximumyield and purity, the crystals must be produced very slowly. This is done byusing urea as a source of ammonia.

Ammonia is a weak base and it can only reactslowly. Therefore, the formation of the calcium oxalate monohydrate crystalscan be slowed down. The percentage of Ca bymass in the unknown sample of CaCO3 was determined to be 31.58% inthis lab while the actual percentage is 30.88%. This percentage of 31.58% isdifferent from the prediction of 68.

74% because a lot of HCl were added onpurpose at that point to keep the pH of the solution under 2. Therefore, there wasan excess amount of HCl in the solution. Hence it is unreasonable to use thisamount of HCl to predict the amount of Ca in the unknown sample.  The final numbers of31.

67%, 31.37% and 31.70% are quite alike with a standard deviation of 0.149 andthe relative error is only 2.27%.

The percent of calcium was found to be 0.7%more than the actual percentage. This error can be explained by some accidentsthat occurred during the lab. The crucibles were not handled properly using thekimwipes some times and this can cause dirt to stick on to the crucibles andcausing the final measured mass to be bigger than it should be. The mass of thesintered crucibles and the crucibles with the calcium oxalate monohydrate werenot measured using the same electronic balance – this can cause the final datato be a little bit off than expected.

Also, calcium oxalate monohydrate ishydroscopic – it is very likely that the crystals absorbed some water from thesurroundings during the weighing and drying process and result in a larger massto be measured and recorded. Overall, this lab was verysuccessful as the standard deviation of the data is only 0.149 and the relativeerror of the data collected is only 2.27% which is very small.

The techniquesof gravimetric analysis and homogeneous precipitation were studied andsuccessfully utilized in this lab. The percentage of calcium (31.58%) in anunknown sample of impure calcium was successfully determined by converting thecalcium into CaC2O4H2O through gravimetric analysis.  AcknowledgementsI discussed the role ofpH in this experiment with Yingjie Ji from BB1. Reference1.    Seo, D.

K., Park, S. S.

, Hwang, J., & Yu, T. (2010). Study of thepyrolysis of biomass using thermo-gravimetric analysis (TGA) and concentrationmeasurements of the evolved species. Journal of Analytical and AppliedPyrolysis, 89(1), 66-73. doi:10.

1016/j.jaap.2010.05.

0082.    Dunn, E., & Robson, P. (1965). Quantitatively gravimetric analysisof fatty ester mixtures by thin-layer chromatography.

Journal ofChromatography A, 17, 501-505. doi:10.1016/s0021-9673(00)99901-13.    Li, Z.

, & Kobayashi, N. (2005). Determination of MoistureDiffusivity by Thermo-Gravimetric Analysis under Non-Isothermal Condition. DryingTechnology, 23(6), 1331-1342. doi:10.1081/drt-2000595234.

    Chem 203 lab manual.

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