Weight Relations

• Atomic Mass and Atomic Weight

  • The actual mass of an atom is very small. A hydrogen atom, for example, has a mass of only 1.67 x 10-24 grams. This is inconveniently small number and not a very practical one to use. To simplify chemical measurement, chemist use a relative mass system for the elements. The system is based on the mass of the carbon isotope known as carbon-12. This isotope has been arbitrarily assigned a value of 12 atomic mass units. Thus, an atom with a mass twice as great as that of a carbon atom would have an atomic- mass number of 24.
• The mass number of an element is based on the total number of protons and neutrons in an atom of that element. Using a carbon atom an atomic mass of 12 as our example, the following chart shows how the number of protons and neutrons determines the mass number. The carbon atom has six protons and six neutrons these gives us the mass number of the carbon atom.
• Atoms of an element having the same number of protons- and therefore the same atomic number- but different numbers of neutrons- and hence different mass numbers-are called ISOTOPES.
• It is very important to note that all isotopes of hydrogen have the same atomic number, or number of protons.
• Atomic Weight and the Mole: Gram Atomic Weight.

  • The balances in a chemistry lab measures not atomic weight but in grams. So a system has been developed for expressing atomic weights in grams, and we call this weight as gram-atomic weight. Do not confuse gram-atomic weight with atomic weight. They are two entirely different concepts.
• The gram atomic weight of an element is simply the number of grams of an element numerically equal to its atomic weight.
• Another important concept in chemistry is a measure called mole. A mole is an amount of substance containing the same number of elementary units (atoms, molecules, electron, or ions) as there are atoms in exactly 12 g of carbon-12. (the exact number of atoms in 12 g of carbon-12 has been found to be 6.02 x 10^23, a number is known as Avogadro’s Number. The gram-atomic weight of any element contains exactly 1 mole of atoms of that elements.
• Avogadro’s Number (6.02 x 10^23) of atoms = 1 mole of atoms = 1 gram-atomic weight
• The mole is used to represent amounts of many particles, including atoms, ions, electrons, and molecules. Just as a quart of water represent 32 oz of water, a mole of atoms represent 6.02 x 1023 atoms.
• It is often necessary to know the number of moles of atoms represent by given weight of an element. We use mathematical equation to arrive at the number of moles. The steps involved in using equation are listed below.

  Step 1. Write the formula of the math equation:

  number of moles = given weight of the elements in grams/weight of 1 mole (of the element)

  Step 2. List the data needed to solve the equation.

  Step 3. Substitute the data into the formula.

  Step 4. Do the calculation indicated.
• Molecular Weight

  Just as we can find the atomic weight of an element, we can find the molecular weight of a compound. The molecular weight of a compound is found by adding up all of the atomic weights in the compound.
• Molecular Weight and the Mole: Gram- Molecular Weight

  We can express molecular weight in grams (gram molecular weight) just as we express atomic weight in grams (gram-atomic weight). And just as 1 gram-atomic weight is equivalent to 1 mole of atoms of an element, 1 gram-molecular weight is equivalent to 1 mole of molecules of a compound.
• The General Mole Concept

  • The term mole is used in chemistry to describe the same number of units of atoms, molecules, formula units, ions or electrons. The number of units in a mole is Avogadro’s Number 6.02 x 10^23.

  1 mole of atoms = 1 gram atomic wt. = 6.02 x 10^23 atoms

  1 mole of molecules = 1 gram molecular wt. = 6.02 x 10^23 molecules

  1 mole of formula units = 1 gram formula wt. = 6.01 x 10^23 formula units

  1 mole of ions = 1 gram ionic wt. = 6.02 x 10^23 ions.

  A mole of any substance is 6.02 x 10^23 units of that substance.
• Formulas for calculating the Number of Moles of a substance.

  1. Number of moles = w.t in grams of the atoms/wt. of 1 mole (of the atom)
  2. wt. Number of moles of molecules = wt. in grams of the molecules/wt. of 1 mole (of the molecule)
  3. Number of moles of any substance = Number of units of substance/6.02 x 1023 units/mole