CHM 111 Empirical Formula Fall18 - online.docx - Empirical...

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Empirical Formula - Online Objectives To determine the empirical formula of a compound by use of a combination reaction. To practice techniques for obtaining and recording precise mass values. To learn safe practices of Bunsen burner use. Introduction The empirical formula of a compound is the simplest (smallest) whole-number ratio of moles of elements in the compound. Determination of the empirical formula requires three steps: Obtain the actual mass of each element in the compound Calculate the moles of each element from its actual mass and molar mass Express the ratio of moles of each element as small whole numbers Example Elemental analysis of a sample of an unknown crystalline solid reveals the presence of 5.52 g of sodium and 8.48 g of chlorine as the only components. The moles of each element are calculated: Moles of sodium: 5.52 g Na x 1 mol Na 22.99 g Na = 0.240 mol Na Moles of chlorine: 8.48 gCl x 1 molCl 35.45 gCl = 0.239 molCl The mole ratio of sodium to chlorine in the compound is 0.240 : 0.239. Expressing the mole ratio as the simplest whole-number mole ratio reveals 1 mol Na : 1 mol Cl. The empirical formula is NaCl , and the compound is identified as sodium chloride (table salt). The empirical formula of a metal oxide can be determined by carrying out a combination reaction at high temperature in air. The mass of the metal before the reaction, and the total mass of the metal oxide after the reaction are measured. From the difference between the masses, the mass of oxygen in the metal oxide is determined. The moles of each element are calculated and the mole ratio of the two elements yields the empirical formula for the metal oxide. Example A 5.90 g sample of titanium metal combines with oxygen in the air to form 9.84 g of titanium oxide. The empirical formula of the titanium oxide can be determined as follows: Moles of titanium: 5.90 gTi x 1 molTi 47.87 gTi = 0.123 mol Ti Mass of oxygen: 9.84 g compound – 5.90 g Ti = 3.94 g O Moles of oxygen: 3.94 gO x 1 molO 16.00 gO = 0.246 molO Empirical Formula Fall18 1
The mole ratio of titanium to oxygen in the compound is 0.123 : 0.246. Expressing the mole ratio as the simplest whole-number mole ratio reveals 1 mol Ti : 2 mol O. The empirical formula is TiO 2 , and the compound is identified as titanium(IV) oxide . SPECIAL PROCEDURE : Bunsen Burner Use and Safety Introduction: Bunsen burners are used in the chemistry laboratory as a source of heat which can be used for simple heating, sterilization or combustion. A burner is designed to allow gas and air to mix in a controlled manner. Typically, natural gas (methane) is the fuel source used. A Bunsen burner (Figure 1) consists of rubber tubing that is connected to a gas control valve. When the gas valve is turned on the gas flows into the Bunsen burner via the gas inlet. The needle valve at the bottom of the Bunsen burner is used to regulate the flow of gas. The air vent is used to regulate the amount of oxygen that flows into the burner.

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