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Empirical Formulas: Flashcards

Revise empirical and molecular formulas and the steps to find them.

9 cardsIntermediate

9 cards

  1. Question
    What is an empirical formula?
    Answer

    The simplest whole-number ratio of the atoms in a compound.

  2. Question
    What is a molecular formula?
    Answer

    The actual number of each kind of atom in one molecule of a compound.

  3. Question
    What is the empirical formula of glucose, CX6HX12OX6\ce{C6H12O6}?
    Answer

    CHX2O\ce{CH2O} (divide every subscript by 6)

  4. Question
    What is the empirical formula of hydrogen peroxide, HX2OX2\ce{H2O2}?
    Answer

    HO\ce{HO}

  5. Question
    What are the steps from percent composition to an empirical formula?
    Answer

    Assume 100 g (% → g), divide by atomic masses (g → mol), divide by the smallest number of moles, then make whole numbers.

  6. Question
    Your mole ratio comes out as 1 : 1.50. What should you do?
    Answer

    Multiply both by 2 to get 2 : 3. Never round 1.5 to 2.

  7. Question
    How do you get the molecular formula from the empirical formula?
    Answer

    Find nn = molar mass ÷ empirical formula mass, then multiply every subscript by nn.

  8. Question
    A compound has empirical formula CHX2\ce{CH2} and molar mass 42.08 g/mol. What is its molecular formula?
    Answer

    n=42.08÷14.03=3n = 42.08 \div 14.03 = 3, so CX3HX6\ce{C3H6}.

  9. Question
    Why can't you use the percentages directly as the atom ratio?
    Answer

    Percentages compare masses. Atoms of different elements have different masses, so you must convert to moles first.