MODULE 7

• The percentage of something is essential to everyday life
• Nutrition label of processed foods indicates the percentage of substances that is in the product
• Percentage composition is a piece of important information used in monitoring health conditions
• Percentage composition


  The ratio of the atomic mass of an element in a compound and the total mass of all elements in the compound multiplied by one hundred
• Percentage composition

  • 21%
  • 37%
  • 42%
  • 56%
• Percentage composition of water (H2O)

  • H=16%;O=84%
• Ways to apply knowledge on percentage composition


  • Identifying the element present
  • Checking if the substance is pure
  • Knowing the value of the substance
  • Knowing the amount of sodium in instant noodles
• Percentage composition of calcium carbonate (CaCO3)

  • 40%
• Empirical formula


  The simplest whole-number ratio of the elements present in a chemical compound
• Molecular formula


  The formula that contains the actual number of atoms in a chemical compound
• Empirical and molecular formulas


  • C4H10 (molecular)
  • P4O6 (molecular)
  • C6H12 (molecular)
  • P2O5 (empirical)
• Molecular formula of a compound with 65.45% C, 5.48% H, 29.08% O, and molar mass of 110.0 g

  • C6H6O2
• Empirical formula of a compound with 47.37% C, 10.59% H, 42.04% O

  • C3H9O3
• Molecular formula of a compound with empirical formula C4H5N2O and molar mass of 194 g/mol

  • C8H10N4O2
• One mole of aluminum foil is not equal to one mole of tin foil
• The particle that represents a covalent compound is a formula unit
• The atomic mass of an element is not the same as its molar mass
• The molar mass of water is 18 g/mol
• Avogadro's number is used to convert the number of moles to the number of representative particles
• The oxoacids of chlorine contain the elements H, Cl, and O
• The percent composition of the oxoacids of chlorine varies
• The mass of chlorine in the oxoacids of chlorine can be calculated using the periodic table
• Perchloric acid (HClO4) is the best source of chlorine among the oxoacids of chlorine
• The empirical formula has the lowest whole number ratio of atoms
• The empirical formula is not always different from the molecular formula
• The empirical formula shows the simplest whole-number ratio of elements, while the molecular formula shows the actual number of atoms
• if the total percentage of all elements is equal to 100
• Steps in calculating the empirical formula of a compound from its percentage composition


  1. Convert the percentage of each element to an actual mass in grams

  2. Convert the mass of each element to its number of moles

  3. Divide the computed mole of each element by the smallest mole and round off the quotient if it is not a whole number

  4. Use the quotient to represent the number of atoms of each element in the compound
• Steps to determine the molecular formula

  1. Divide the actual molar mass of the compound by the empirical mass

  2. Multiply the quotient with the subscripts of the element in the empirical formula
• Empirical Formula


  The simplest whole number ratio of atoms of each element present in a compound
• Finding Empirical Formula


  1. Determine the mass percentage of each element

  2. Convert the mass percentages to moles

  3. Divide the moles by the smallest mole value to get the simplest whole number ratio

  4. Write the empirical formula using the simplest whole number ratio
• Molecular Formula


  The actual number of atoms of each element present in a compound
• Finding Molecular Formula

  1. Determine the empirical formula

  2. Determine the molar mass of the compound

  3. Divide the molar mass by the empirical mass to get the number of empirical formula units

  4. Multiply the subscripts in the empirical formula by this number to get the molecular formula
• Percentage composition is the ratio between the mass of an element and the total mass of the compound, multiplied by 100
• The compound with the greatest percentage of sodium is sodium hydroxide (NaOH)
• Finding Empirical Formula from Percent Composition

  1. Convert the percent composition to grams assuming 100g total

  2. Divide the grams of each element by its atomic mass to get the moles

  3. Divide the moles by the smallest mole value to get the simplest whole number ratio

  4. Write the empirical formula using the simplest whole number ratio
• The empirical formula of lactic acid is CH2O, and its molar mass is 90 g/mol, so its molecular formula is C3H6O3
• The empirical formula CH is not a valid molecular formula
• The empirical formula C2H3 is not a valid molecular formula
• The empirical formulas C2H4O2 and C6H12O6 are valid molecular formulas