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Homework Help: How many moles of electrons are in 1mL of water?

  1. Apr 11, 2017 #1
    1. The problem statement, all variables and given/known data

    How many moles of electrons are in 1mL of H2O

    2. Relevant equations

    1mL = 1g

    moles to grams conversion - moles x molar mass = grams

    mass of 1 electron = 9.109382 x 10-28g

    moles to molecules = number of moles x avogadro's number (6.022140857 × 1023)

    3. The attempt at a solution

    Molar mass of H2O
    H = 1g, O = 16g, H2O = 18g

    Number of electrons in 1 molecule of H2O
    H = 1e−, O = 8e−, H2O = 10e−

    Moles in 1g of H2O = 1g/18g = 0.0556M

    Molecules in 0.0556M of H2O
    0.0556M x (6.022140857 × 1023) = 3.348310835 x 1022 molecules

    Number of electrons in 0.0556M of H2O
    (3.348310835 x 1022) molecules x 10e− = 3.348310835 x 1023e−

    Moles of electrons in 0.0556M of H2O
    (3.0501042450753967 x 104g) of e− / 6.022140857 × 1023 = 0.5556M
  2. jcsd
  3. Apr 11, 2017 #2


    User Avatar

    Staff: Mentor

    Final answer looks OK (although it contains way too many significant digits).

    Is it really needed?

    No need to use that. Instead of doing several conversions (mass → moles of water → number of molecules → number of electrons → moles of electrons) you can directly calculate number of moles of electrons in known number of moles of water. Just like there are ten electrons per molecules of water there are ten moles of electrons per each mole of water, so just multiplying number of moles of water by ten will give the correct answer (in other words: mass → moles of water → moles of electrons).
  4. Apr 11, 2017 #3
    That makes so much more sense! I'll definitely use this approach next time.
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