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17
Calculate the molecular formula of compound S.
Step 1: Calculate the E.F.M (Empirical Formula Mass).
= 12+2(1)+16
= 28+2
= 30
Step 2: Divide its Molecular Mass by its E.F.M.
= 60/30
= 2
Step 3: Multiply the answer in Step 2 with its Empirical Formula.
= 2 (CH2O)
= C2H4O2
• Therefore the molecular formula for this compound is C2H4O2
• Relative molecular (formula) mass
• The symbol for the relative molecular mass is Mr and it refers to the total mass of the
molecule.
• To calculate the Mr of a substance, you have to add up the relative atomic masses of all the
atoms present in the formula.
The Mole & Avogadro's Constant
• Chemical amounts are measured in moles.
• The mole, symbol : mol, is the SI unit of amount of substance. It is often denoted by n.
• One mole of a substance contains the same number of the stated particles, atoms,
molecules, or ions as one mole of any other substance
• One mole contains 6.02 x 1023 particles (atoms, ions or molecules). This number is known
as the Avogadro Constant. For example,
• One mole of sodium (Na) contains 6.02 x 1023 atoms of sodium.
• One mole of hydrogen gas (H2) contains 6.02 x 1023 molecules of hydrogen.
• One mole of sodium chloride (NaCl) contains 6.02 x 1023 formula units of sodium
chloride.
• The mass of 1 mole of a substance is known as the molar mass.