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Chemical Formulas

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What this exam is like

The Chemical Formulas exam has 30 multiple-choice questions across these levels: Secondary (30). Each attempt draws 10 at random from the level you choose. You get two tries per question, and each answer is explained afterwards. At the end you see your grade.

Sample questions

  1. 1. In the formula H₂O, what does the small 2 mean?

    • Hydrogen weighs 2
    • There are 2 hydrogen atoms in the molecule
    • There are 2 oxygen atoms
    • There are 2 water molecules
    Show the answer

    ✅ There are 2 hydrogen atoms in the molecule

    The subscript affects ONLY the symbol before it and counts atoms within one molecule. The number of molecules goes in front, full size: 2 H₂O is two water molecules.

  2. 2. What is the formula of methane?

    • C₂H₄
    • CH₂
    • CH₃
    • CH₄
    Show the answer

    ✅ CH₄

    Carbon has four hands and fills them with four hydrogens. It is the natural gas burnt in kitchens.

  3. 3. What is the formula of calcium oxide (quicklime), if calcium has valency 2?

    • Ca₂O
    • Ca₂O₃
    • CaO
    • CaO₂
    Show the answer

    ✅ CaO

    Calcium 2 and oxygen 2: crossing them would give Ca₂O₂, but formulas are always written simplified, so CaO. That simplifying step is the one most often forgotten.

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