Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: If the Atoms Were Not Neutral

  1. Feb 1, 2005 #1
    Because the charges on the electron and proton have the same absolute value, atoms are electrically neutral. Suppose this were not exactly true, and the absolute charge of the electron were less than the charge of the proton by (1.50e-5)e.

    now its a 4 part question, i think i can do the other 3, but i don't understand the first part.

    1) What would be the net charge in 1.50 liters of water?

    (Take the density of water to be 1 g/cm3 and the molecular weight of water to be 18.)

    so i solve for the number of moles, and then the number of molecules. and i get 55.55 moles or 3.35x10^25. from there i'm just not sure what i should do. My first thought is to solve for how many protons/electrons in each molecule. I don't happen to have the periodic table handy, but i'm sure i could find it online. But usually he gives us all that sort of info, so i was sorta doubting thats what i had to do. I was thinking maybe there was some easier way. Any ideas?
  2. jcsd
  3. Feb 1, 2005 #2
    figure out how many of each atom are there for example there is one Oxygen for each water molecule, so there is 1 x (# of molecules of water) Oxygen atoms,
    Then you can look at the periodic table and find out how many protons/electrons an Oxygen atom has...

    can you figure out the rest?

    edit: looking over your calculations thus far, they are incorrect, post your work
    keep in mind 1 cm^3 = 1*10^-3 L , and molar mass is 18 g/mol
    Last edited: Feb 1, 2005
  4. Feb 1, 2005 #3
    ok ya, thats what i was thinking i'd do, but i dunno, i thought that there may be a different way to do it. Awell. A second question: i'm not sure what (1.50e-5)e. means.
  5. Feb 1, 2005 #4
    I think the first 'e' is actually supposed to be a capital 'E'. This is another way of writing 10^x ... Calculators use this notation often. The second 'e' stands for the magnitued of the charge of an electron which is 1.6022*10^-19 C.
  6. Feb 1, 2005 #5
    by the way,, your calculations are wrong so far... post your work so we can figure out what went wrong.
  7. Feb 1, 2005 #6
    i figured actually. but the second e? just like a unit, or shouldi multiply by the real charge?

    secondly, i forget how to use the periodic table. helium has 1 proton and one electron, i forget how i know about oxygen. is it 8 because thats oxygens atomic number. 8 protons 8 electrons i mean??
  8. Feb 1, 2005 #7
    my bad... i did it for one liter. So its 83.33 moles and 5.02x10^25 molecules. 5.02x10^25 oxygen atoms and 1.00x10^26 hydrogen atoms
  9. Feb 1, 2005 #8
    just substitute the charge of an electron for e.
    so (1.5E-5)e = (1.5*^10-5)(1.6022*10^-19)C

    I can't tell by the way the question is worded whether that is the charge of the new electron, or if were supposed to subtract that amount from the charge of a proton. hmm... I think its the latter.

    Yes exactly...
    Last edited: Feb 1, 2005
  10. Feb 1, 2005 #9
    Yep looks good :smile:
  11. Feb 1, 2005 #10
    oops, An easier solution would be to figure out the "new" charge of a water molecule rather than to calculate it for each atom,,, but both methods work just as good, sorry
  12. Feb 1, 2005 #11
    new charge of the water molecule? as in??
  13. Feb 1, 2005 #12
    "new" as in taking account of the changed value for the charge of an electron that they gave you. Sorry if this confused you... I was just saying that its quicker to calculate the charge of the water molecule rather than calculate the number of atoms, but do it whichever way is easiest for you. If you wan't I can show you what I was talking about when your done.
    Last edited: Feb 1, 2005
  14. Feb 1, 2005 #13
    go ahead and show me now, i've done the problem 4 times.
  15. Feb 1, 2005 #14
    1 H20 molecule has 2 Hydrogen atoms which contribute 1 electron/proton each,
    1 H20 molecule has 1 Oxygen atom which contrubues 8 electrons/protons each,

    so each water molecule has a total 10 electrons/protons.
    Normally the charge of an electron equals the charge of a proton so the charges balance out and the molecule is neutral.
    But in this case, the charge of the electron is less than that of a proton by (1.5E-5)e, and since a proton is + , and an electron is -, this means each molecule will have a net positive charge
    since there are 10 electrons/protons in a H20 molecule, this means that 1 H20 molecule has a net charge of,
    10 * (1 - 1.5*10^-5) * (1.6022*10^-19) C = 1.602*10^-18C

    Since there is a total of 5.02*10^25 H20 molecules, the net charge of the whole thing is,
    (5.02*10^25) * (1.602*10^-18)C = 8.043*10^7 C
    Last edited: Feb 1, 2005
  16. Feb 1, 2005 #15

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Just work out the number of protons by dividing the mass of water by the mass of a proton:[itex]1.5 kg/1.67(E-27) = 8.9E26[/itex] protons

  17. Feb 1, 2005 #16
    Unfortunately that doesn't work because Oxygen has as many neutrons as protons, and the mass of a neutron is the same as that of a proton, so your number calculates approximately twice the number of protons.
    Last edited: Feb 1, 2005
  18. Feb 2, 2005 #17

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Right you are. This gives you the number of nucleons. So to find the number of protons or electrons, multiply by 10/18.

    BTW, you can see that the charge is enormous. The force between two 1.5 kg blocks of water one metre apart would be
    [tex]F = kqQ/R^2 = 9E9*8E7*8E7/1 = 5.76E25 N.[/tex] . That is enough to accelerate the earth (6E24 kg) at rate of 10 m/sec^2.

    Last edited: Feb 2, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook