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EXPERIMENT 2 : Determination of The Formula Unit Of A Compound Objetive : 1. To synthesis a zinc chloride compound 2. To determine the formulae unit of Zinc Chloride Theory : In this experiment, we prepared a simple compound composed of zinc and chlorine. Once the mass of Zinc and the mass of the compound is known, the mass of the Chlorine can be determined. Using these mass, the percentage composition of the product can be determined and the formulae unit can be calculated. Apparatus : • Hot plate • Glass rod • White tile • Crucible tongs • Crucible ( 50 ml) • Analytical balance • Measuring Cylinder • • 1.
Chemical reagent : 6M HCl Zinc powder Procedure: The crucible is weight and the exact mass of the crucible was recorded. 2. Approximately 1.25g of Zinc was placed on the crucible. The crucible with its content is weight and the exact mass of Zinc is determined. 3. 10ml of 6M HCl is added carefully into the crucible containing the Zinc powder and the content is stir gently using a glass rod. A vigorous chemical reaction occurs and hydrogen gas was released. 4. 5ml of HCl is added at a time ,if the Zinc powder does not dissolved completely until all the Zinc is dissolved. The amount of acid used is not exceeded 20 ml 5. The crucible is placed on a hot plate in the fume cupboard and the content is heated slowly so that the compound does not splatter during the heating process. 6. The compound is heated gently until it is completely dry. Make sure that the compound does not melt. 7. The crucible was allowed to cool at room temperature and then it is weight. 8. The crucible is reheat. The crucible was cooled to room temperature and it was weight again. The procedure is repeated until the difference in mass does not exceed 0.02g.
9. The mass of Zinc Chloride from the final weight of the sample was determined. The mass of the Chlorine in Zin Chloride is calculated. 10. The formula unit of Zinc Chloride was calculated. Result :
1.
5.
Mass of crucible (g) Mass of crucible + Zinc powder (g) Mass of Zinc powder (g)
82.06 82.31 0.25
Mass of crucible + Zinc chloride (g) Mass after 1st heating Mass after 2nd heating Mass after 3rd heating
82.62 82.60 82.59
Mass of Zinc Chloride (g) Mass of Chlorine (g)
0.53 0.28
Discussion: Calculation to determine the formula unit ZnCl2 2. The mass of solvent is larger than the mass of solid. To attain a more accurate result, the compound is cool to room temperature so that the HCl would evaporate to the surrounding leaving Zinc Chloride itself in the crucible. Hence, we are only weighing Zinc Chloride itself in the crucible. In addition, the mass of the compound is more accurate. 3. Balance chemical equation : Zn + 2HCl ZnCl2 + H2 4. Actual formula : ZnCl2 Experimental formula : ZnCl2 The experiment result is equal to the theoretical result , in this case thr formula of the compound. Possible source of errors: I. Zero errors is one of the most error. This error is due to the reading of the measuring apparatus that is not at nil value when the sucstance is measured. II. The eye position of the observer might be perpendicular to the scale of measuring apparatus, and hence resulting in an inaccurate reading or measurement. III. During the mixing of the mixture, some unwanted or foreign substance might be mix together with the zinc and hydrochloric acid and hence resulting in an impure compound. Due to this fact, the mass measured might be not the mass if Zinc Chloride solely.
IV. While heating the compound, excessive heat might be appl to the compound causing it to melt. When the compound melts, the compound might be react with other substance resulting a product other than Zinc Chloride. 6.
7.
Step of precaution : • The reading of every measuring apparatus should be set to zero as to prevent zero error. • The observer should make sure that the eye position of the observer should be perpendicular to the scale of measuring instrument. • The experiment should be carry out in a control environment . The fan should be switch off so that wind do not deviate the measurement of the measuring apparatus. • The observer have to make sure that the Zinc powder dissolved fully into hydrochloric acid so to make sure that all the zinc react with Hydrochloric acid. • Conclusion: • ZnCl2 is synthesis in this experiment • The formula unit of Zinc Chloride is ZnCl2
