Question

It's not actually a new question, it's just that this site somehow has yet to allow comments on questions posted by other people.

For this answered question,

http://www.chegg.com/homework-help/questions-and-answers/1535-g-methanol-ch3oh-burned-constant-volume-bomb-calorimeter-water-temperature-rose-2027o-q2011605?trackid=21378677&strackid=193b0f5b&autosuggest=1&ii=1&uqry=When 2.548g of methanol was burned in a constant-volume bomb calorimeter, the water temperature rose from 21.25 degrees celcius&event=click_submit#

the person who answered it used 26.87-20.27 as the change in temperature for both the water AND the calorimeter. But, since the calorimeter clearly has a different specific heat since it's made of metal and we don't even know its mass, how did "Anonymous" get away with the assumption that the change in temperature of the water was the exact same change in temperature as the calorimeter?

Furthermore, they said "heat given by calorimeter = 1.75x1000x ( 26.87 - 20.27)
= 11550 J"

But heat capacity isn't specific heatt. the heat capacity of the calorimeter is 1.75 joules per degree celcius, and NOT joules per gram-degree-celcius. And yet, here ananymous is using heat capacity as though it's equivalent to specific heat, so their answer he typed isn't right, it's in joules but according to their own equation, its ACTUALLY in joule-grams.

Answer 1

Yes, you are right about the calorimeter. However the problem state that is a constant volume bomb, and you have a mass of water surrounding the calorimeter, so you can easily assume that the difference of temperature of water could be the same of the calorimeter, because the calorimeter is gaining a heat, along with the water. It's true, it's best to know the mass of the calorimeter, but in this case, you don't need it. With the assumption of T, you can easily calculate the heat of the calorimeter. For the other part, the 1000 he use was to convert the kJ into J, and it's not the 1000 g of water, so his answer is in fact in Joule and not Joule-grams. but to get clear with this question, I'll do the problem here.

As I say before, the difference of T is the following: 26.87 - 20.27 = 6.6 °C

The heat capacity of the calorimeter: 1.75 kJ/°C so Q = Cp*T

Qc = 1.75 kJ/°C x 6.6 °C = 11.55 kJ or 11,550 J

Now the heat given by water: Qw = m*Cp*T

Qw = 1000 g x 4.184 J/g °C x (6.6 °C)

Qw = 27614.4 J

The total heat would be: Qt = 27614.4 + 11550 = 39164.4 J or 39.164 kJ

Finally with the molecular weight of CH₃OH : 12 + 4(1) + 16 = 32 g/mol

moles of Methanol = 1.535 g / 32 g/mol = 0.048 moles

H = Q/n

H = 39.164 kJ / 0.048 moles

H = 815.9 kJ/mol and to the change it would be -815.9 kJ/mol.