Key for 2005 USNCO National Exam, Part III 2005 U.S. NATIONALCHEMISTRY OLYMPIADKEY FOR NATIONAL EXAM – PART IIILab Problem 1You have been given three beakers containing NaHCO3(sodium hydrogen carbonate), CaCl2(calcium chloride), and tap water. These two compounds react in the presence of water.Propose a balanced chemical equation to account for this reaction, and support your proposalwith all possible qualitativeand quantitativeobservations and measurements.This problem requires the students to carry out a reaction and make qualitative and quantitativeobservations.1. Sample answer:CaCl2+ 2NaHCO3→Ca(OH)2(s) + 2 CO2(g)+ 2 Na+(aq) + 2 Cl–(aq)orCaCl2+ 2NaHCO3→CaCO3(s)+ CO2(g) + H2O + 2 Na+(aq) + 2 Cl–(aq)Weigh 1 mmol CaCl2and add to one corner of the plastic bag, then weigh 2 mmol NaHCO3andadd to the other corner of the plastic bag. Add water to the NaHCO3corner without mixing, sealthe bag, and weigh. Allow the reagents to mix, observing for bubbling and inflation of the bag(to verify CO2production) and formation of a white insoluble solid (to verify either Ca(OH)2orCaCO3). Carefully expel the excess gas and reweigh the bag to measure the amount of CO2lost,and determine whether that mass corresponds to one or two moles per mole of calcium. The pHof the final mixture will be measured as well; the first reaction should be close to neutral (both aweak base and a weak acid are formed), while the second should be noticeably acidic (from theCO2).An excellentanswer will clearly note both qualitative and quantitative observations that will bemade, and will indicate how they will be interpreted to provide evidence of the nature of thereaction.An averageanswer will note qualitative or quantitative observations that will be made, but willnot have clear indications of how those observations will be used.
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- Spring '13
- Chemistry, Qualitative Research, sodium bicarbonate, Calcium carbonate, Solvay process