Gravimetric exercises (2007) Question 1 Different quantities of nitrogen oxide (NO) are listed below. Which one contains the least number of molecules? A. 6 × 102 L at 273 K and 1 atm B. 6 × 1023 molecules C. 6 × 102 g D. 6 mol Click here for solution Question 2 When 2.54 g of solid iodine reacts with excess chlorine and the unreacted chlorine is evaporated, 4.67 g of a yellow product remains. The empirical formula of the product is A. ICl2 B. ICl3 C. ICl4 D. ICl5 Click here for solution Some rocks were thought to consist of insoluble silica (SiO2) and calcium carbonate ( CaCO3; molar mass 100.1g mol1). The fraction of CaCO3 in an 8.64 g sample of the crushed rock was determined by mixing the sample with excess hydrochloric acid. The acid reacts with CaCO3 according to the following equation. 2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g) The resulting solution was filtered and the SiO2 that was collected was washed and dried. The mass of SiO2 was found to be 1.55 g. a. Calculate the expected percentage of CaCO3 in the original rock sample.   Click here for solution b. In order to check the result from part a., excess ammonium oxalate solution was added to the filtered solution. The calcium ions present precipitate as CaC2O4.H2O The CaC2O4.H2O was collected by filtration, washed and dried. It was then heated to convert it to CaO (molar mass 56.1 g mol1) and a mass of 3.87 g was obtained. Using this mass of CaO, calculate the percentage of CaCO3 in the rock sample. Ensure that you express your answer to an appropriate number of significant figures.   Click here for solution c. Suppose that the percentage of CaCO3 determined by the chemical analysis with ammonium oxalate was less than the result found in part a. above. Provide one possible explanation for the difference. Click here for solution Continue with more gravimetric analysis exercises from past exam papers