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Transcript
Atomic Masses The most accurate method of comparing masses of atoms is by using the mass spectrometer. The mass spectrometer knocks electrons off the atoms or molecules being analyzed and changes them into positive ions. An applied electric field then accelerates these ions into a magnetic field. An accelerating ion produces its own magnetic field, which tends to change the path if the ion. The amount of path deflection for each ion depends on its mass. The atomic mass of any element is a mixture of isotopes, thus the atomic mass we use for that element is an average value reflecting the average of the isotopes composing it. (%of isotope1)(Massofisotope1)+(%ofisotope2)(Massofisotope2)+… Most elements occur in nature as mixture of isotopes, thus atomic weights (masses) are usually average values. The masses for each element listed in the table inside the front cover of your chemistry textbook are an average value based on the isotopic composition of the naturally occurring element. The mole's numeric value is 6.022x1023 One mole of something consists of 6.022x1023 units of that substance. The mass of 1 mole of an element is equal to its atomic mass in grams. Molar Mass The molar mass of a substance is the mass in grams of one mole of the compound, and is found by adding the average atomic masses of each atom in the compound. Percent Composition of Compounds We can obtain the mass percent of the elements from the formula of the compound by comparing the mass of each element present in 1 mole of the compound to the total mass of 1 mole of the compound. Empirical Formula Determination (simplest whole number ratio) • Since mass percentage gives the number of grams of a particular element per 100 grams of compound, base the calculation on 100 grams of compound. Each percent will then represent the mass in grams of that element • Determine the number of moles pf each element present in 100 grams of compound using the atomic masses of the elements present • Divide each value of the number of moles by the smallest of the values. If each resulting number is a whole number (after appropriate rounding), these numbers represent the subscripts of the elements in the empirical formula • If the numbers obtained in the previous step are not whole numbers, multiply each number by an integer so that the results are all whole numbers Molecular Formula Determination (actual number of atoms present.) • Obtain the empirical formula • Compute the mass corresponding to the empirical formula • Calculate the ratio Molar mass Empirical formula mass • The integer from the previous step represents the number of empirical formula units in one molecule. When the empirical formula subscripts are multiplied by this integer, the molecular formula results. This procedure is summarized by the equation: Molecular formula = (emp. formula) x (Molar Mass/Emp. Form.Mass) Chemical Equations A chemical change involves a reorganization of the atoms in one or more substances. By a chemical equation with the reactants on the left side of an arrow and the products on the right side Bonds have been broken, and new ones have been formed. It is important to recognize that in a chemical reaction, atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the products. By balancing a chemical equation, there must be the same number of each type of atom on the product side and on the reactant side of the arrow. A chemical equation often gives the physical states of the reactants and products: State Symbol Solid s Liquid l Gas g Dissolved in water (in aqueous solution) aq The relative numbers of reactants and products in a reaction are indicated by the coefficients in the balanced equation Writing and Balancing the Equation for a Chemical Reaction • Determine what reaction is occurring. What are the reactants, the products and the physical states involved? • Write the unbalanced equation that summarizes the reaction described above • Balance the equation by inspection, starting with the most complicated molecules. Determine what coefficients are necessary so that the same number of each type of atom appear on both reactant and product sides. Do not change the identities (formulas) of any of the reactant or products Calculating masses of reactants and products in chemical reactions: --Balance an equations for the reaction. --Convert the given quantity or quantities to moles. --If there is more then one given reactant, determine which is limiting reactant. Divide moles by coefficient…smaller is limiting) --Use the balanced equation to convert moles of limiting reactant or given quantity to moles of desired substance --Convert to the units of desired substance.