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Homework Help: Total charge of e in a given volume of water

  1. Jan 21, 2005 #1
    this is killing me.
    the question ask for total charge ina given volume of water.

    -first convert the volume to m^3 from cm^3 since i know the density of water at 4 degree cel (most dense) is 1000kg/m^3.

    -then imultiply the m^3 by 1000kg/m^3. then convert it to grams.

    - divide by 18 grams (mass of H2O). which will gives us # of moloe of H2O. then multiply by 6.02*10^23 to get the amount of molec of H2O.

    -Multiply by 10 electron (since each H2O have 10 e total).

    -then lastly multiply it by 1.6*10^-19 C (charge of an electron)

    but still i keep gettign the wrong answer. anyone know why?
  2. jcsd
  3. Jan 21, 2005 #2
    are you asking the number of e or charge of total e? your reasoning is completely fine...gimme the origina question and the model answer... your hw might asking you a completely different thing...
  4. Jan 21, 2005 #3
    here the original question:

    What is the total charge on all the electrons in 21.10cm3 of water?
  5. Jan 21, 2005 #4
    and your answer is....
  6. Jan 21, 2005 #5
    i got 1128126.79 C
  7. Jan 21, 2005 #6
    i got 1.29*10^6, what is your model answer
  8. Jan 21, 2005 #7
    i don't ahve an answer...since its online HW which will give u a "correct" or "incorrect" statment. n both our answer is wrong.
  9. Jan 21, 2005 #8
    my answer should be right... so do yours... check your arithematics ...
  10. Jan 21, 2005 #9
    err.can u give me ur full answer? then i can check if its right.

    basically i get from 21.10cm^3 to 21.1 grams

    -21.1 g / 18 g = mol of h2O

    -mol of h2o * 6.02x10^23 = molec of h2o

    -molec of h2o * 10 = total e

    -total muitliply by 1.6x10^-19 = 1129084.44 (is differ from my last answer caz i used 18 g this time instead of the very precise one)
  11. Jan 21, 2005 #10
    sry.. i put 24.1 instead of 21.1 in my calculator.... my calculation is exactly the same as you...try 1.13*10^6 or something alike... you calculation is absolutly fine...
  12. Jan 21, 2005 #11
    nah.i got 3 try left :( can't waste it.

    the system basically detect if more/less digit is require.
  13. Jan 21, 2005 #12


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    21.10 cm^3 is 21.10 g

    21.10g * (1 mol / 18.01 g) = 1.172 mol

    1.172 mol * (6.02 x 10^23 molecules / 1 mol) = 7.05 x 10^23 molecules.

    There are indeed ten electrons per water molecule. So we have:

    7.05 x 10^24 electrons

    (7.05 x 10^24 electrons ) * (1.60 x 10^-19 C / 1 electron) = 1 128 457.524 C

    = 1.13 X 10^6 C

    That is closer to Bailey's answer

    *EDIT* no...after Vincent's correction, we all got the same answer. Your system is wack.

    *EDIT* 2: Actually, Bailey, notice that our answers don't match beyond 3 sig dig accuracy. Yet, that should be sufficient for a final answer! :grumpy: Part of the discrepancy is because you used 18 g/mol the second time, but even your first answer doesn't match mine. Did you carry all digits until that last step in the calculation? How many digits does the dreaded computer want in the answer anyway?
    Last edited: Jan 21, 2005
  14. Jan 21, 2005 #13
    omg.i tried it ( 1.13 X 10^6 C ).

    n its still wrong. 2 try left now.......lol

    edit: reply to edit post from above:

    well.i know the system will tell me if i need more digit or if i used the wrong unit.
    Last edited: Jan 21, 2005
  15. Jan 21, 2005 #14
    the answer is -1.13E+06 C!!!!!!!!!!!!! damnit...b/c i use the constant at the text which doesn't state a negative sign.......so.................doh! lol lol lol...............
  16. Jan 21, 2005 #15


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    U should have seen that from the beginning...Electrons have negative charge...

  17. Jan 21, 2005 #16
    what can i say....... i can be a complete idiot sometime.......lol

    btw......thx for the reply guys.
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