lilsis76
  • lilsis76
A 100.0 mL sample of water is heatedto its Boiling Point. How much heat (in kj) is required to vaporize it? (Assume Density of 1.00 g/mL). Please Help. Word problems freak me out, and I really dont understand the concept of conversion if someone can help me understand and answer this problem please. THank you.
Chemistry
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SOLVED
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katieb
  • katieb
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aaronq
  • aaronq
if the water is already at 100 celsius, then you only need to multiply the mass or moles (depending on what the \(\Delta H_{vap}\) given is in units of. q=\(\Delta H_{vap}\) *m orq=\(\Delta H_{vap}\) *n
lilsis76
  • lilsis76
@aaronq but I dont get it, so OH! wait boiling point of water is at 100 degree Celcius. so I multiply by H=1.008 and O=16 ?
aaronq
  • aaronq
hm no you're confusing things. Boiling point and molar masses aren't related like that. DId they give you a value for \(\Delta H_{vap}\)?

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lilsis76
  • lilsis76
all i have from the quesiton is:.... 100.0mL sample of water heated to its boiling point. ?___in kilojoules. Density of 1.00 g/mL. And in the book it has...... liquid water, T=20 deg. Celcious, Density is .998 g/cm cubed at 1 atm on page 456
lilsis76
  • lilsis76
oh, that was from the 3 states of water in chap. 11
lilsis76
  • lilsis76
FOUND IT!
lilsis76
  • lilsis76
okay, page 472. Heats of vaporization at their boiling points and at 25 deg. C. Waters normal boiling point is 100 deg cel. heat of vap(kJ/mol) at 25 deg C is 44.0
lilsis76
  • lilsis76
is that what I need?
aaronq
  • aaronq
You need it to be at 100 celsius, it's \(\Delta H_{vap}\)= 40.7 kJ/mol (usually in a test they give you this, if not memorize it)
aaronq
  • aaronq
so all you need to do is to convert the water given to grams, then to moles. \(\rho=\dfrac{m}{V}\); where, \(\rho\)=density then, \(n=\dfrac{m}{M}\) then, q=\(\Delta H_{vap}*n\) make sure they units are the same.
lilsis76
  • lilsis76
okay, sorry could you watch me on here to see if i do it right. okay so D=m/vol. Density=\[\frac{ 18g }{ 100mL }\]
lilsis76
  • lilsis76
is that right? @aaronq
aaronq
  • aaronq
where'd you get 18 g from? what you're solving is for mass
aaronq
  • aaronq
ohh, m is not M, they're not asking for molar mass 1.0 g/mL =\(\dfrac{m}{100 mL}\)
lilsis76
  • lilsis76
ya, I added he H2O together. = 18
aaronq
  • aaronq
that's molar mass (M) not mass (m)
aaronq
  • aaronq
yeah, little details, like capitals, make a big difference in chemistry.
lilsis76
  • lilsis76
oh... :? then im completely lost
aaronq
  • aaronq
rearrange: \(m=\rho*V\) m=1 g/mL * 100 mL = 100 g now, convert to moles (use the molar mass this time)
lilsis76
  • lilsis76
okay so....here hold on
lilsis76
  • lilsis76
okay so....here hold on \[d=\frac{ m }{ v } \]
aaronq
  • aaronq
volume should be a capital "V", lower case v means velocity
lilsis76
  • lilsis76
okay D=\[\frac{ m }{ V }\]
aaronq
  • aaronq
haha density should be either "rho" (\(\rho\)) or lower case d :P, but don't worry about it THIS time.
lilsis76
  • lilsis76
now i .....well okay, we have the mass that is the 100grams, the volume is going to be the... 100 mL-->Liters right? so we divide by 1000 giving me 0.1 L
aaronq
  • aaronq
hmm we already used that equation, remember? we found mass using m=\(\rho*V\)= 100g now we need to find moles: \(n=\dfrac{m}{M}\)
lilsis76
  • lilsis76
okay so n= mass being the 100g / M-Molar mass? being 18 grams 100g/18g =5.56?
lilsis76
  • lilsis76
@aaronq did i do it right this time?
aaronq
  • aaronq
yep thats right. molar mass is in units of "g/mol" though.
aaronq
  • aaronq
as in "18 g is equal to 1 mole of water"
lilsis76
  • lilsis76
oh okay hold on let me write that down
lilsis76
  • lilsis76
so then the answer @aaronq is 5.56 moles of kJ is needed to heat the sample?
aaronq
  • aaronq
nope, you can't express energy in terms of moles :P q=n*\(\Delta H_{vap}\) and you're done
aaronq
  • aaronq
q=heat
lilsis76
  • lilsis76
lol let me try
lilsis76
  • lilsis76
i cant do it..............:(
aaronq
  • aaronq
lol why not? q=n*ΔHvap =5.56 moles * 40.7 kJ/mol = 226.292 kJ
lilsis76
  • lilsis76
because im crying thats why. haha its sad
aaronq
  • aaronq
lol this shouldn't be that bad.. :P are you planning to study chemistry further?
lilsis76
  • lilsis76
yes. I took chemistry 101 and passed with a C, took chem 102 but was in a car accident. returned to school and had to take a failing grade in order to retake it, I took it again and I failed. I took it again and I withdrew cuz I realized I was rushing myself to get back into school of 2 years not taking chemistry. and Now I kind of know it but I dont get conversions and I have no idea what all the equaitons and conversion factors are. do you know of a website that has them?? I really wanna be a biology major to be a doctor
lilsis76
  • lilsis76
sorry about my sad life story but Im so stressed
aaronq
  • aaronq
sorry to hear that :S um a website with conversion factors? no particularly, but if you google whatever specific conversion you want, it'll usually pop up in yahoo questions or something.
lilsis76
  • lilsis76
its okay, thank you. And yes ive done that. but I think I might need it for test taking :/ I dunno. ugh...thank you for helping me out.
lilsis76
  • lilsis76
Im going to close this problem. would you mind helping me with one more problem, of many homework quesitons that I dont get? @aaronq
aaronq
  • aaronq
You should make a list of all the equation as you go along. and yeah, no problem. I have to go in a few minutes so please post it fast :P
lilsis76
  • lilsis76
okay. and I have so many questions but right now its why cant i stop sniffling. haha :')
aaronq
  • aaronq
haha dont stress yourself out, it doesn't help! :P calm and collected is where it's at.
lilsis76
  • lilsis76
i know, i know. Im trying so hard. BLAH! haha

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