
#1
Jan2105, 12:02 AM

P: 43

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
Jan2105, 12:17 AM

P: 611

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...




#3
Jan2105, 12:20 AM

P: 43

here the original question:
What is the total charge on all the electrons in 21.10cm3 of water? 



#4
Jan2105, 12:23 AM

P: 611

total charge of e in a given volume of water
and your answer is....




#6
Jan2105, 12:27 AM

P: 611

i got 1.29*10^6, what is your model answer




#7
Jan2105, 12:30 AM

P: 43

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.




#8
Jan2105, 12:35 AM

P: 611

my answer should be right... so do yours... check your arithematics ...




#9
Jan2105, 12:44 AM

P: 43

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) 



#10
Jan2105, 12:48 AM

P: 611

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...




#11
Jan2105, 12:50 AM

P: 43

nah.i got 3 try left :( can't waste it.
the system basically detect if more/less digit is require. 



#12
Jan2105, 12:53 AM

Emeritus
Sci Advisor
PF Gold
P: 5,198

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! 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? 



#13
Jan2105, 12:56 AM

P: 43

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. 



#14
Jan2105, 02:03 AM

P: 43

OMG!!!!!!!!
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............... 



#15
Jan2105, 02:09 AM

Sci Advisor
HW Helper
P: 11,863

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


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