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Mathematics of Chemical Formulas

Mathematics of Chemical Formulas. Objectives: determine the atomic mass of elements and the formula mass of compounds determine the gram atomic mass of elements and the gram formula mass of compounds Calculate the mass of a given number of moles of a substance

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Mathematics of Chemical Formulas

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  1. Mathematics of Chemical Formulas Objectives: determine the atomic mass of elements and the formula mass of compounds determine the gram atomic mass of elements and the gram formula mass of compounds Calculate the mass of a given number of moles of a substance Calculate the number of moles in a given mass of a substance Calculate the number of molecules in a given number of moles of a substance Calculate the volume of a given mass of a gaseous substance at STP Determine the percentage composition of a substance from its formula Determine the empirical formulas and molecular formulas of compounds. Created by C. Ippolito Dec 2006

  2. Stoichiometry • study of the quantitative relationships that can be derived from chemical formulas and chemical equations • chemical formulas • represent substances • chemical equations • represent reactions between substances Created by C. Ippolito Dec 2006

  3. Formula Mass • Formula Mass (molecular mass) • sum of the atomic masses of all of the atoms represented by the formula • Find the Formula Mass of Ca(OH)2 • Element Mass Atoms Total • Ca 40 amu x 1 = 40 amu • O 16 amu x 2 = 32 amu • H 1 amu x 2 = 2 amu • 74 amu Created by C. Ippolito Dec 2006

  4. Gram Atomic Mass • Gram atomic mass • that quantity of an element that has a mass in grams equal to its atomic mass • Gram formula mass • that quantity of a substance that has a mass in grams equal to its formula mass • Gram molecular mass • equivalent to formula mass for molecular substances Created by C. Ippolito Dec 2006

  5. Mole • 6.02 x 1023 atoms (a.k.a Avogadro’s number) • # of atoms in a gram atomic mass of any element • # of formula units in a gram formula mass of any substance • gaseous substances at STP [0oC and 1 atm (101.3 kPa)] • has a volume 22.4 liters Created by C. Ippolito Dec 2006

  6. Percentage Composition • the percentage by mass of each element in a compound • determined by experimentation Created by C. Ippolito Dec 2006

  7. Formula Determination • Formula determined if one knows: • mass of each element in a sample • or • the percentage composition • Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula? Created by C. Ippolito Dec 2006

  8. Formula Determination from Masses • Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula? • Find mole ratio: • nitrogen  oxygen Created by C. Ippolito Dec 2006

  9. Formula Determination from Percentage Composition • Example: What is the empirical formula of a compound with a percentage mass of 65.2% arsenic and 34.8% oxygen by mass? • Find mole ratio: • arsenic  oxygen Created by C. Ippolito Dec 2006

  10. Empirical Formula (con’t) Created by C. Ippolito Dec 2006

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