these might sound like silly questions but im freaking out please help (1 Viewer)

[Jazzerina]

freaking out slightly!!! i'm not very good at chemistry and am having trouble recalling how to work the molar heat of combusion for a substance (at the moment its ethanol) if anyone could please help me out that would be great.

also how do i find the molecular weight of a substance??

i know these might seem like silly questions but please help asap

 

[Azreil]

Okay.

Delta H = - mCdeltaT

m=mass of water
C=specific heat capacity of water [4.18]
delta T= change in temp ie larger temp - lower temp.

The molecular weight is found by adding the mass numbers of a substance (from the periodic table) together. eg Molecular weight of CO2 is 12 + 2 * 16 = 44. A mole of a substance is it's molecular weight in grams.

 

[Jazzerina]

i still couldnt get the right answer. the question is

A student used the apparatus shown to determine the molar heat of combustion of ethanol.

can containing 300g of water

The following results were obtained.
​

Initial mass of burner 133.20 g
Final mass of burner 132.05 g
Initial temperature of water 25.0°C​

Final temperature of water 45.5°C
What is the molar heat of combustion calculated from this data?

 

[what else?]

I got -25.707

 

[anythingworks]

After you find delta H you divide it by the number of moles

delta H = -mCdeltaT = - 0.300 x 4.18 x 10^3 x 20.5 = - 25707 J = - 25.707 kJ

n = mass / molar mass = 1.15 / 46.068 = 0.025

delta H / n = - 25.707 / 0.025 = -1028.28 kJ mol

 

[Undermyskin]

Just so you know that when expressing molar heat, write it without any + or - sign.

 

[crazypanda]

After you find delta H you divide it by the number of moles

delta H = -mCdeltaT = - 0.300 x 4.18 x 10^3 x 20.5 = - 25707 J = - 25.707 kJ

n = mass / molar mass = 1.15 / 46.068 = 0.025

delta H / n = - 25.707 / 0.025 = -1028.28 kJ mol

where did u get 1.15 from for the mass???

 

[anythingworks]

where did u get 1.15 from for the mass???

Initial mass of burner - Final mass of burner

133.20 - 132.05 = 1.15 g