Question

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.

Answer 1

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

Answer 2

@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 ?

Answer 3

hm no you're confusing things. Boiling point and molar masses aren't related like that. DId they give you a value for \(\Delta H_{vap}\)?

Answer 4

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

Answer 5

oh, that was from the 3 states of water in chap. 11

Answer 6

FOUND IT!

Answer 7

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

Answer 8

is that what I need?

Answer 9

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)

Answer 10

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.

Answer 11

okay, sorry could you watch me on here to see if i do it right. okay so D=m/vol.
Density=\[\frac{ 18g }{ 100mL }\]

Answer 12

is that right? @aaronq

Answer 13

where'd you get 18 g from? what you're solving is for mass

Answer 14

ohh, m is not M, they're not asking for molar mass

1.0 g/mL =\(\dfrac{m}{100 mL}\)

Answer 15

ya, I added he H2O together. = 18

Answer 16

that's molar mass (M) not mass (m)

Answer 17

yeah, little details, like capitals, make a big difference in chemistry.

Answer 18

oh... :? then im completely lost

Answer 19

rearrange:

\(m=\rho*V\)

m=1 g/mL * 100 mL = 100 g

now, convert to moles (use the molar mass this time)

Answer 20

okay so....here hold on

Answer 21

okay so....here hold on \[d=\frac{ m }{ v } \]

Answer 22

volume should be a capital "V", lower case v means velocity

Answer 23

okay D=\[\frac{ m }{ V }\]

Answer 24

haha density should be either "rho" (\(\rho\)) or lower case d :P, but don't worry about it THIS time.

Answer 25

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

Answer 26

hmm we already used that equation, remember?

we found mass using m=\(\rho*V\)= 100g

now we need to find moles: \(n=\dfrac{m}{M}\)

Answer 27

okay so
n= mass being the 100g / M-Molar mass? being 18 grams

100g/18g =5.56?

Answer 28

@aaronq did i do it right this time?

Answer 29

yep thats right. molar mass is in units of "g/mol" though.

Answer 30

as in "18 g is equal to 1 mole of water"

Answer 31

oh okay hold on let me write that down

Answer 32

so then the answer @aaronq is 5.56 moles of kJ is needed to heat the sample?

Answer 33

nope, you can't express energy in terms of moles :P

q=n*\(\Delta H_{vap}\)

and you're done

Answer 34

q=heat

Answer 35

lol let me try

Answer 36

i cant do it..............:(

Answer 37

lol why not?

q=n*ΔHvap =5.56 moles * 40.7 kJ/mol = 226.292 kJ

Answer 38

because im crying thats why. haha its sad

Answer 39

lol this shouldn't be that bad.. :P

are you planning to study chemistry further?

Answer 40

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

Answer 41

sorry about my sad life story but Im so stressed

Answer 42

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.

Answer 43

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.

Answer 44

Im going to close this problem. would you mind helping me with one more problem, of many homework quesitons that I dont get? @aaronq

Answer 45

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

Answer 46

okay. and I have so many questions but right now its why cant i stop sniffling. haha :')

Answer 47

haha dont stress yourself out, it doesn't help! :P calm and collected is where it's at.

Answer 48

i know, i know. Im trying so hard. BLAH! haha