NAME: Sonia Najib COURSE: CHEM 371, M/W Lab EXPERIMENT: Gravimetric Determination of Calcium (as a CaC2O4•H2O) DATE OF LAB: 30th January 2013 INTRODUCTION The purpose of this lab was to find the average molarity and average weight percent of calcium in an unknown solid that was provided to students. The experiment also exposed students to the gravimetric analysis technique that quantified the unknown substance while keeping the students aware about the chemistry behind crystallization and precipitation of the substance. Gravimetric analysis is basically a method that helps scientists determine “the amount of analyte through the measurement of mass” (Yoder, 2012). This method is observable by crystallization or precipitation of the analyte. The experiment aimed at making calcium precipitant in “large, easily filtered, relatively pure crystals of product” by carrying out a slow precipitation. A slow precipitation indicates “particle growth dominates over nucleation” (Harris, 2012). MATERIALS i. ii. iii. iv. v.
medium-porosity, sinteredglass funnels (3) Vial of Unknown #6 T/Th Exp #2 Dessicator Vacuum suction apparatus Side-arm filter flask
vi. vii. viii. ix. x.
25mL transfer pipet w/ rubber bulb 100mL volumetric flask glass rod w/ rubber policeman microwave oven hot plate
PROCEDURE i.
Three sintered-glass funnels were cleaned and rinsed thoroughly before usage. TECHNIQUE: The funnels were, individually, placed on a side-arm filter flask that was already attached to a vacuum suction apparatus via a thick tube. The vacuum suction apparatus was checked to make sure that it was working before performing the filtration. A beaker containing distilled water was also prepared beforehand. Once everything was set up, the vacuum was turned on, and the funnel was filled with the distilled water. The distilled water passed through into the filter flask indicating that the funnel was working well. Same test was done to the other two funnels
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab ii.
iii.
iv. v. vi. vii. viii. ix. x.
xi.
The funnels were dried by microwaving them using a 900-W kitchen microwave oven provided in the lab. The funnels were dried in two heating periods of 4 min and 2 min with 15mins of cooling down time in between heating. The funnels were weighed on the electronic balance after each 15min-cooling period, before the next round of heating period. These steps stopped once the weights reach to a constant measurement within 0.3mg. A dessicator was chosen to ensure that the funnels were further dehydrated and was also used to store the funnels. Paper towels were used to handle the funnels. A vial of solid CaCO3 unknown was provided to every student to make an unknown solution. To make the solution, 1.5775g of the unknown were weighed on the electrical balance (4 repetitive weighing) and transferred into the calibrated 100mL volumetric flask. 3.8mL of 12M HCl was added and the flask was filled up to the 100mL point with distilled water. A small portion of the unknown solution was used to rinse a 25mL transfer pipet. The washings were discarded. Exactly 25mL of unknown was transferred to three 250mL beakers. Each beaker was diluted with 75mL of 0.1M HCl. 5 drops of methyl red indicator was added to the each beaker. 25mL ammonium oxalate was added to each beaker while stirring with a cleaned glass rod. The rod was removed and rinsed into the beaker with small portions of distilled water. 15g of urea were added to each sample and each beaker was covered with a watchglass, and boiled gently until the indicator turned pale yellow. Each solution was filtered through a weighed funnel using suction. 3mL of ice-cold water was added to the beaker and a rubber policeman was used to help transfer the remaining solid to the funnel. The procedure was repeated with small portions of ice-cold water until all precipitate has been transferred to the funnel. For the final rinsing, two portions of 10mL ice-cold water were poured over the precipitate. The precipitate was dried with vacuum suction, and then dried in the microwave exactly as in the beginning.
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab RESULTS i.
Data:
Funnel # 20 404 480 mass of empty sintered glass funnel 20.929 20.8083 20.5793 mass of funnel + samples 21.4553 21.3291 21.097 difference/mass of CaC2O4•H2O 0.5263 0.5208 0.5177 2+ mass of Ca in 25mL 0.14436 0.14285 0.14200 wt% 0.58% 0.57% 0.57%
ii.
Calculation to find the wt% of calcium in 25mL Eg. Funnel #20 Density of water at 24C = 0.997296 g/ml Mass of water at 24C = 24.9324 g Mass of Ca2+ in 25mL = Mass of CaC2O4•H2O x MM Ca2+/MM CaC2O4•H2O = 0.5263g x (40.0784g/mol / 146.1130g/mol) = 0.14436g wt%
iii.
= mass of Ca2+ in 25mL/mass of soln. x 100% = 0.14436g/24.9324 g x 100% = 0.57%
Calculation to find the molarity of Ca2+ in 25mL Eg. Funnel #20 # of moles of Ca2+ = Mass of CaC2O4•H2O/ MM CaC2O4•H2O = # of moles of CaC2O4•H2O = 0.5263g/146.1130g/mol = 0.003602 moles of CaC2O4•H2O = 0.003602 moles of Ca2+ Molarity = moles of Ca2+/L = 0.003602/0.025L = 0.1441 M
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab
DISCUSSION Based on the calculations method in the Result section, the wt% for the calcium unknown # 6 for T/Th lab was 2.30%. One the other hand, the molarity of the calcium unknown was 1.428M.
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab APPENDIX I Raw Data Empty taret boat Trial 1 1.5775 Trial 2 1.5761 Trial 3 1.5760 Trial 4 1.5760
Mass of sintered glass funnels (g) Funnel #20 Funnel # 404 Funnel # 480 20.9766 20.8575 20.5943 20.9293 20.8085 20.5799 20.9293 20.8079 20.5795 20.929 20.8083 20.5795 20.929 20.8083 20.5793 Mass of samples after being in the dessicator ~5days Funnel #20 Funnel # 404 Funnel # 480 21.4553 21.3289 21.0959 21.4553 21.329 21.0958 21.4551 21.3293 21.0963 21.4556 21.3294 21.0966 21.4553 21.3291 21.0970
APPENDIX II Mass of CaC2O4•H2O Sintered-glass funnels mass of empty funnel mass of funnel + samples difference/mass of CaC2O4•H2O
Finding wt % of Ca2+ Sintered-glass funnels 20 Mass of Ca2+in 25mL 0.1444 Average mass Mass of Ca2+in 100mL wt%
0.1431 0.5723 2.30%
20 20.929 21.4553 0.5263
404 0.1429
404 20.8083 21.3291 0.5208
480 20.5793 21.0970 0.5177
480 0.1420
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab Finding Molarity of Ca2+ Sintered-glass funnels #moles of Ca2+ Molarity
20
404
480
0.003602 0.1441
0.003564 0.1426
0.003543 0.1417
Average Molarity 0.1428 Average #moles of Ca2+ 0.003570 Molarity of Ca2+ 1.428
APPENDIX III
NAME: Sonia Najib COURSE: CHEM 371, M/W Lab SOURCES Harris, D.C. (2013). Exploring Chemical Analysis 5th Edition. New York: W. H. Freeman and company Yoder, C. (2012). Wired chemist. Retrieved from http://www.wiredchemist.com/chemistry/instructional/laboratorytutorials/gravimetric-analysis
