Empirical formula of substance

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Empirical formula of substance

  1. 1. Experiment 4 : Empirical Formula of a CompoundSource: (2008) Introductory Chemistry Laboratory. Minneapolis Community and Technical College. Retrieved fromhome.minneapolis.edu/~naughtwe July 2010.Objectives: • To prepare a compound from a weighed quantity of metal. • To determine the empirical formula of the compound.Introduction:Chemical compounds are composed of atoms of two or more elements chemically combined in definiteproportions. The atoms in a compound are held together by chemical bonds. The total mass of eachelement in the compound depends on the number of atoms involved, and therefore, the elements are indefinite proportions by mass. The ratio of moles of the constituent elements in the compound is nearlyalways a ratio of small, whole numbers. The formula containing the lowest possible whole number ratiois known as the empirical formula.To find the empirical formula we must combine the elements to generate the compound under conditionsthat allow us to determine the mass of each element. From these data the moles of each element may bedetermined. By dividing the moles obtained for each element by the smallest number of moles, we obtainquotients that are in a simple ratio of integers, or are easily changed to such a ratio. An example of thistype of calculation is shown below.Sample calculations:A strip of aluminum weighing 0.69 g is ignited yielding an oxide that weighs 1.30 g. Calculate theempirical formula of the compound formed.A. Calculate the mass of oxygen that reacted to combine with the aluminium: Mass of oxide = mass of aluminium + mass of oxygen Therefore: Mass oxygen = mass oxide – mass aluminium = 1.30g – 0.69g = 0.61gB. Now that you have the masses, you can calculate the moles of each element: Moles = mass/ molar mass Moles aluminium = 0.69g/ 27.0gmole-1 = 0.026 mole Moles oxygen = 0.61g/16.0gmol-1 = 0.038 moleC. Obtain the ratio of atoms by dividing the moles of each by the smallest number of moles obtained from step B. In this case the moles of aluminium (0.026 mole) is smaller than the moles of oxygen (0.038mole). Al:O = (0.026/0.026) : (0.038/0.026) = 1 : 1.5 The ratios need to be in whole numbers. Multiply out the values to get rid of any fraction and get the least whole number ratio. Al:O = 2:3 Use the ratio to obtain the formula. Formula = Al2O3
  2. 2. In today’s experiment, you will determine the empirical formula of the oxide of magnesium formed byheating magnesium metal in air. You will use an analytical balance to weigh all the masses. You mustnote which balance you used for your first measurement and use the same balance for all massmeasurements. You will be using a porcelain crucible (30mL size) to heat the magnesium.Procedures:CAUTION: Read before starting the procedures. a) If the crucible seems dirty, do not scrub to clean it, as you may accidentally scratch and weaken it. b) Do not touch the crucible with your fingers as the oil and dirt from your fingers can add to the mass measurements. Always handle the crucible with tongs. c) While transporting the crucible, hold it with the tongs, and place it on a small beaker, to avoid accidentally dropping it. d) Always light and adjust the Bunsen burner to a blue flame, prior to placing the crucible into the triangle. e) All heating and cooling must be done in stages, as will be described by your instructor. f) Never weigh any object while it is hot!Exercise 1 A. Preparation of crucible and magnesium:Preparing the Crucible 1) Weigh the crucible and its cover and record the mass (to the closest . 001g). 2) Place the covered crucible on the clay triangle* and heat for three minutes. Move the burner and allow the crucible to cool (5-10 min). When the crucible is cool (no heat felt emanating when fingers when 0.5 cm in away) transfer the crucible and cover to the balance using tongs and reweigh. If you cannot reweigh right away, you can temporarily place the crucible on a wire gauze. (Remember – do not touch the crucible with your hands or you may transfer grease and dirt to the container. 3) Record the mass of the crucible and its cover. If the mass is off by +/- 0.001 g from the mass recorded in step 1, repeat step two. (* For this all exercises in this lab, do not heat crucible while it is on the wire gauze. The gauze will insulate it, preventing adequate heating.)Preparing the Magnesium 4) Obtain a piece of magnesium ribbon from your instructor. Clean the magnesium, over a trash bin, with sandpaper to remove any oxide from the surface until the surface displays the lustrous sheen of the pure metal. 5) Make a loose ball of the magnesium and place it at the bottom of the crucible. It should fit into the bottom 2/3 of the crucible. (Winding the magnesium metal on a pen or pencil with a pocket clip is a good way to start. Remember your goal is to expose as much of the ribbon to air as possible so do not make the ball too tight or it may take longer to burn.) 6) Cover the crucible then weigh the crucible along with the magnesium ribbon and record the mass. 7) Calculate the mass of the metal -it should be between 0.180-0.220g.Exercise 1B. Burning the magnesium in air 1) Replace the crucible (containing the magnesium ribbon) on the clay triangle with lid at an angle leaving a crack open. During the heating process, observe the magnesium metal through the crack and record your observations. Heat the crucible slowly at first by moving the Bunsen burner
  3. 3. underneath the crucible (make sure the flame is blue) and continue to heat strongly for at least 10 minutes. 2) Toward the end of the heating (do not turn off the burner yet), carefully remove the lid using the tool provided. If the magnesium flares up with a bright yellow flame, the reaction is not complete. Replace the lid and continue to heat until all the metal has been reacted and the reaction appears to be complete. (No metal should be apparent; the contents should be white or slightly grey.) Record your observations. 3) Turn down the flame and after one minute, remove the burner and turn it off. 4) Allow the crucible to cool to room temperature gradually while positioned on the clay triangle.Exercise 1C. Converting any nitride formed to oxide:The air around us is composed mainly of oxygen (~21%) and nitrogen (78%). Since magnesium is anactive metal it combines with both oxygen and nitrogen when burnt, forming the oxide (MgO) and thenitride, Mg3N2. The nitride can be converted to the oxide by adding water then reheating the contents. Theequations for the reactions involved are shown below: Mg3N2(s) + 6H2O(l)  3Mg(OH)2 (s) + 2 NH3(g) Mg(OH)2 (s) + heat  MgO(s) + H2O(l) 1) Wet the contents of the crucible by adding about 5-10 drops of distilled water using a dropper. Record any odor you detect after adding the water. 2) Recover the crucible with the lid slightly ajar. Heat gently to vaporize any excess water. After all water has vaporized, continue heating with a strong flame for 8-10 minutes. 3) Turn off the flame. Allow the crucible to cool completely. When cool, weigh the mass of the crucible along with the compound formed (record all of he digits displayed by the balance). 4) Reheat the covered crucible and contents for three minutes. Turn off the flame. Allow the crucible to cool completely. 5) When cool, weigh the mass of the crucible along with the compound formed. If the mass is off by +/- 0.001 g from the mass recorded in step 3, repeat step 4.Exercise II. Calculate the empirical formulaFor qualitative analyses like this one, it is best to repeat the experiment several times and use an average.Your labmates are all doing the same experiment, so their data represent replicates of the experiment.You will first calculate your ratio and obtain an empirical formula based solely on your data, then youwill compare it to what you get using the class data. 1) Using the method shown by the sample calculation in the introduction, determine the empirical formula of the magnesium oxide based on your experimental masses for magnesium and oxygen. Make sure you follow the rules for significant figures. 2) Collect the class data for mass of magnesium and mass of oxygen. Calculate the moles of magnesium and oxygen in each case. Use the calculated moles to make a scatter plot of moles of oxygen (y axis) versus moles of magnesium (x-axis). (Make sure to use an appropriate scale so that the scatter is easy to see.) Obtain a best fit line and an equation for the line with the y- intercept set as zero – your equation will be in the form of y = mx. (The m represents the slope of the line.) This equation can be read as follows: the number of moles of oxygen = (m) x (the number of moles of magnesium) The ratio of the magnesium to oxygen based on this would be 1Mg:mO. Use the ratio 1:m for Mg:O to get the empirical formula of the magnesium oxide.
  4. 4. Pre-lab exercise: Empirical Formula of a Compound (to be submitted before lab session starts)Answer all questions, show all calculations where appropriate. :1. What are the cautionary measures that you should take in handling the crucible in today’s experiment? • If the crucible seems dirty, do not scrub to clean it, as you may accidentally scratch and weaken it. • Do not touch the crucible with your fingers as the oil and dirt from your fingers can add to the mass measurements. Always handle the crucible with tongs. • While transporting the crucible, hold it with the tongs, and place it on a small beaker, to avoid accidentally dropping it. • Always light and adjust the Bunsen burner to a blue flame, prior to placing the crucible into the triangle. • All heating and cooling must be done in stages, as will be described by your instructor.2. In today’s experiment, when magnesium burns in air, in addition to the oxide being formed, what other product will be formed? How do you convert this other compound to oxide? • When magnesium burns in air, the products formed are the magnesium oxideand magnesium nitride. Mg + O2 2MgO + Mg3N2 • Magnesium nitride can be converted to oxide by adding water then reheating the contents. The equations for the reactions involved are shown below: Mg3N2(s) + 6H2O(l)  3Mg(OH)2 (s) + 2 NH3(g) Mg(OH)2 (s) + heat  MgO(s) + H2O(l)3. In this lab activity, you utilized laboratory techniques and skills to determine the formula of magnesium oxide experimentally. However, the end result can be predicted. a) Based on their positions in the periodic table: What is the most likely charge of a magnesium ion? What is the most likely charge of an oxygen ion? • Magnesium is a cation and based on its position on the periodic table, it has an 2+ charge. • Oxygen is an anion and based on its position on the periodic table, it has an 2- charge. b) Based on the charges of magnesium and oxygen ions, what would one predict for the formula of magnesium oxide? • Mg2+ + O2- MgO4. A 3.90 g sample of sodium is allowed to react completely with sulfur to form a sulfide which weighs 6.64 g. Calculate the following: i. mass of sulfur : Mass of sodium + mass of sulfur = mass of sulfide 3.90g + mass of sulfur = 6.64g Mass of sulfur = 6.64g – 3.90g = 2.74g

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