Chem help (1 Viewer)

[ayeplus_]

Hi~

I need help in answering this question:

Calculate the final temperature if a student burnt 0.15g butanol to heat 100mL water at 21°C. The specific heat of the water is 4.18 J °C-1 g-1, and the molar heat of combustion of butanol is 2675.5 kJmol-1

Also, considering the mass (-mCΔT), do you use the total mass of butanol and water? Or do you just use the mass of water?

Thank you

 

[Deliriously]

For molar heat of combustion Qs, you are usually heating up the water so the mass should only be the weight of the water.
To start off, you find how many kJ was released from the combustion. 0.15g of butanol was burnt => (convert that to moles). Then x it by molar heat of combustion of butanol which is 2675.5 kJ per mole. This gives you q

 

[DamTameNaken]

0.15g Butanol = 0.002 mol

0.002 mol = 5kJ

5,000 J = 100 x 4.18 x T
T = 11.96 = 12 (approx)

Final temperature is 33°C.

When doing molar heat equations the mass just refers to the mass of the water as it is only the water that is being heated and the 4.18 J/g/C is a value for water only.

 

[someth1ng]

Although, not directly relevant to the question asked here, it would be worth mentioning that the actual change in temperature will be far smaller due to energy losses. I would also, definitely, recommend knowing at least 3 reasons for this difference.

 

[enoilgam]

*Moved to Chemistry forums*

 

[golgo13]

Although, not directly relevant to the question asked here, it would be worth mentioning that the actual change in temperature will be far smaller due to energy losses. I would also, definitely, recommend knowing at least 3 reasons for this difference.

I agree there's generally a follow up question with "Student X however got this values, which is Y (generally lower than the values you just worked out), give 3 reasons why"