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Unformatted text preview: STOICHIOMETRY
CHEMICAL MEASUREMENTS
Unit 6 /Ch 10 Measurements Atomic mass: The mass of an atom expressed relative to the mass assigned to carbon12. Atomic Mass Unit (amu). Formula Mass: The sum of the atomic masses of all the atoms in a compound. Examples. “The Procedure” Distribute the subscript outside the ( ). List the elements in the compound. List the number of atoms for each. Look up the atomic mass for each and multiply by the number of atoms. Include significant figures and scientific notation as required. Examples. A mole of any element is defined as the number of atoms of that element equal to the number of atoms in exactly 12.0g of carbon12. The number of atoms in one mole of atoms is always the same. Avagadro’s Number (N ) = 6.02 X 1023 A The mass in grams of one mole of a substance is numerically equal to its atomic mass or formula mass in atomic mass units. Examples. The Mole What’s in a mole? Atoms and moles: The number of atoms in one mole of an element is always 6.02 X 1023. Molecules and moles: The number of molecules in one mole of any molecular compound is always 6.02 X 1023. Diatomic Molecules vs. Elements. Formula units and moles: The number of formula units in one mole of an ionic compound is always 6.02 X 1023. Molar Mass The mass in grams of one mole of a substance is called the molar mass of the substance. Molar mass depends on the masses of the particles that make up the substance (atoms, molecules, or formula units). The molar mass of a substance can be determined from its atomic mass or formula mass. “The Mole Concept”
The mole measures both a mass and a number of particles. It is the central unit for converting the amount of a substance from one type of measurement to another. If you know the mass of a given amount of a substance you can calculate the number of moles of the substance. “The Bridge”
Use Avogadro’s # (6.02 E 23) mass
Use molar mass particles MOLE
Use 22.4 L/1 mol volume “Mole Conversions” Identify known and unknown. Select the correct conversion factor. Set up an equation to cancel units. Solve the problem. Sigfig’s, scientific notation, units, and substance. Mole Conversions Mass to moles. Moles to mass. How many moles of NaCl are in 68.9 g? How many grams are in 2.4 moles of LiOH? How many atoms are in 1.5 moles of Calcium? How many moles are in 3.42 E 24 formula units of KBr? Moles to particles. Particles to moles. Moles and Gases Avagadro proposed that at the same temperature and pressure, equal volumes of gases contain the same number of gas particles. One mole of any gas @STP has a volume of 22.4L. This volume of gas is called the molar volume. Used to convert between the volume of gas @ of STP, the # of moles of gas, the # particles in the gas, and the mass of the gas. Practice problems. % Composition The mass of each element in a compound compared to the entire mass of the compound. Solve for the molar mass (GFM or GMM). Solve for each element using the formula: grams of element X 100 molar mass The sum of the percentages should equal 100%. Practice problems. Empirical Formula A formula that gives the simplest whole number ratio of the atoms of the elements. %: Assume 100g and convert all % signs to grams. Convert grams of elements to moles. Divide all the elements by the lowest # of moles to get the ratio. Rounding: Write the empirical formula. Practice problems. 0.010.44 round DOWN 0.450.55 multiply by 2 0.560.99 round UP Empirical Formula
Sample Problems A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H. What is the empirical formula for the compound? Determine the empirical formula of a compound containing 5.75 g Na, 3.5 g N, and 12.0 g O. Molecular Formula The formula that gives the actual # of atoms of each element in a molecular compound. The molecular formula is a multiple of the empirical formula. Procedure: Molar Mass (GMM) Formula Empirical Molecular n(Empirical) n = GMM Molecular GMM Empirical Molecular Formula
Sample Problems Ribose is an important sugar that is found in DNA and RNA. Ribose has a molar mass of 150 g/mol and a chemical composition of 40.0% carbon, 6.67 % hydrogen, and 53.3% oxygen. What is the molecular formula for ribose? ...
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 Fall '08
 petrovich
 Atom, Stoichiometry

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