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To determine the molecular mass of an unknown alkali metal carbonate, X2CO3.

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Introduction

Candidate Name: Candidate Number: Page | International Baccalaureate Diploma Program (IBDP) Session: May 2015 Chemistry HL Lab Report Lab Report Title: To determine the molecular mass of an unknown alkali metal carbonate, X2CO3. Criteria Assessed: * Data Collection and Processing (DCP) * Conclusion and Evaluation (CE) Candidate Name: Candidate Number: International School, Singapore AIM: To determine the molecular mass of an unknown alkali metal carbonate X2CO3 using titration. INTRODUCTION: Since substance Z is an alkali metal carbonate, it can safely be hypothesized that the compound is most likely to be the carbonate of Lithium, Sodium or Potassium since these are the only three alkali metal carbonates which are stable and safe to use in a school laboratory. Alkali metal carbonates are basic in nature and dissolve in water hence forming basic solutions. These basic solutions can readily react with strong acids such as HCl to form a salt and water. Therefore, in order to determine the molecular mass of substance Z, its ability to form alkali solutions was exploited and hence, aqueous samples of substance Z was titrated against 0.1 molarity solutions of HCl. Substance Z reacts with HCl according to the following balanced chemical equation: X2CO3 (aq.) + 2HCl (aq.) ? 2XCl (aq.) + CO2 (g) + H2O (l) VARIABLES: Independent Variables: 1. Mass of substance Z ? The same digital balance was used to weigh out all the nine samples. 2. Volume of substance Z solution ? 25.0cm3 of substance Z solution was used for each of the nine trial. ...read more.

Middle

This data is represented in the following table. 1.5g of substance Z 2.0g of substance Z 2.5g of substance Z Volume of substance Z solution (±0.03cm3) 25.0 25.0 25.0 Average volume of HCl required for neutralization (±0.10cm3) 28.0 37.1 47.2 Table 2: Average volumes of substance Z solution and HCl used in all trials. To determine the molar mass of X2CO3, we need to find the moles of X2CO3 that are present in 25.0cm3 of substance Z solution. Part 1: Determining number of moles of X2CO3 in 1.5g of substance Z Since 1.5g was used to make 250cm3, 25cm3 of solution is assumed to have 0.15g of X2CO3. Number of moles (mol) = Concentration (mol/dm3) x volume (dm3). Therefore, moles of HCl = Concentration of HCl (mol/dm3) × Volume of HCl (dm3) = 0.1 × = 0.0028 moles HCl reacts with substance Z according to the following balanced chemical equation: X2CO3 (aq.) + 2HCl → 2XCl (aq.) + CO2 (g) + H2O (l) Therefore, it can be seen from the balanced equation of the reaction that every two moles of HCl require one mole of X2CO3 which means that 0.0028moles will require 0.0014moles of X2CO3. Since the reaction had reached completion, it can be assumed that the required 0.0014 moles of X2CO3 was present in the 25cm3 of the solution which was assumed to have 0.15g of X2CO3. Therefore, total mass of 0.0014 moles of X2CO3 is considered to be 0.15g. This formula can be rearranged to make molecular mass the subject → . ...read more.

Conclusion

However, over time, evaporation of water from the solution may lead to increase in molarity which in turn would lead to overstatement of the value for the molecular mass of X2CO3. This problem can be solved rather easily by simply making a standardized solution in the lab itself eliminating any dependence on the labels. 3. Parallax error in reading the burette might have led to overstatement or understatement of the value for the molecular mass of X2CO3. This can be avoided by using a mirror against the markings to obtain a more accurate readings and reading only the lower meniscus of the liquid in the burette. 4. Due to the continued usage of the white tile by various students for various experiments, it is unavoidable to have some stains on it. Our white tile had very minute yellowish stain on it which might have interfered with the orange color leading to some inaccuracies in reading the end-point of the reaction. Nonetheless, as much care as possible was taken but a very minor systematic error was unavoidable. Overall, I would say that the experiment was satisfactory in the given conditions where both time and resource were limited. Alternate methods to obtain the molecular mass of unknown substance Z could be used in addition to this method after which, the results can be analyzed to give a much more accurate value for the molecular mass of X2CO3. However, in the time constraints of our lab classes, it was not possible to perform both experiments with the same level of accuracy and we chose not to perform two experiment because that would compromise with the accuracy of both. ...read more.

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