MODULE 6

• Every day you do tasks that require counting and measuring
• Measurement is important to chemists
• Chemists have devised a way to count particles with the unit of mole
• This module contains one lesson: Mass and Mole
• After going through this module, you are expected to

  • Use the mole concept to express the mass of substances
  • Differentiate the molecular unit from the formula unit and compute for the molecular mass
  • Define mole and calculate the number of moles of given compounds
  • Compute for the number of particles given the mass or number of moles
  • Convert the number of moles, mass, and number of particles from one unknown to another
• Molecular unit


  Represents a covalent compound
• Formula unit


  Represents an ionic compound
• Formula mass


  The sum of the atomic masses of each atom in the molecule
• Molecular mass


  The sum of the atoms in one formula unit
• Mole


  A unit used to count particles like atoms and molecules
• One mole is equal to 6.02 x 10^23 particles
• Substances with an equal number of moles


  Have equal mass, equal number of particles, the same amount of energy, and equal volume per unit mass
• Covalent compounds equivalent to 1 mole

  • 18 g H2O
  • 32 g O2
  • 80 g CO2
  • 160 g SO2
• 1.204 x 10^24 formula units of sodium chloride (NaCl) is equal to 2 moles
• 40 g NaOH vs 40 g NaCl


  NaCl contains more number of formula units
• 1.5 moles of glucose (C6H12O6) is equal to 270 g C6H12O6
• 0.5 g of ascorbic acid (C6H8O6) taken daily for 15 days is equal to 2.56 x 10^23 molecules C6H8O6
• 0.5 g of paracetamol (C8H9NO2) taken every 4 hours for 3 days is equal to 0.04 mol C8H9NO2
• Ionic compounds have high melting and boiling points, and are good conductors of electricity in solution and in molten state
• Covalent compounds


  Have low melting and boiling points, and are poor conductors of electricity
• Oxygen is an element, sodium ion is an ion, sodium chloride is an ionic compound, and water is a covalent compound
• Atom vs Ion


  An atom is electrically neutral, an ion has a positive or negative charge
• Ionic compound


  Represented by a formula unit
• Covalent compound


  Represented by a molecular unit
• The atomic mass or atomic weight is the weighted average mass of all isotopes of an element
• The molecular mass is the sum of the atomic masses of each atom in the molecule, and the formula mass is the sum of the atoms in one formula unit
• The molecular mass of water (H2O) is 18 g
• Formula mass


  The mass of a chemical compound calculated from the atomic masses of its constituent elements
• Computing molecular mass or formula mass of compounds


  1. Write the chemical formula

  2. Identify and count the number of atoms for each element

  3. Determine the atomic mass of each element using the periodic table

  4. Multiply the number of atoms of each element to its rounded-off atomic mass, then add the products
• Subscripts represent the number of atoms of an element. When there is no subscript written, it is understood that the element has only one atom in the formula
• Steps to compute molecular mass of water


  1. Write the chemical formula

  2. Identify and count the number of atoms for each element

  3. Determine the atomic mass of each element using the periodic table

  4. Multiply the number of atoms of each element to its rounded-off atomic mass, then add the products
• The molecular mass of water is 18 g
• Steps to compute formula mass of magnesium hydroxide

  1. Identify and count the number of atoms for each element

  2. Determine the atomic mass of each element using the periodic table

  3. Multiply the number of atoms of each element to its rounded-off atomic mass, then add the products
• The formula mass of magnesium hydroxide is 58 g
• Mole


  The amount of substance that contains the same number of atoms as 12.00 grams of Carbon-12 (12C). A mole contains 6.02 x 10^23 particles called Avogadro's number
• Representative particles and their number of moles

  • Atom (1 mol 12C = 6.02 x 10^23 atoms 12C)
  • Ion (1 mol Na+ = 6.02 x 10^23 ions Na+)
  • Ionic compound (1 mol NaCl = 6.02 x 10^23 formula units NaCl)
  • Covalent compound (1 mol H2O = 6.02 x 10^23 molecules H2O)
• Calculating the number of atoms in a given number of moles

  Multiply the number of moles by Avogadro's number
• Calculating the number of moles from a given number of particles

  Divide the number of particles by Avogadro's number
• Molar mass


  The substance's mass in one mole. For elements, it is equivalent to the atomic mass. For compounds, it is equivalent to the molecular mass or formula mass
• Molar mass contains 6.02 x 10^23 particles