Question

When wood is burned, the combustion of cellulose (empirical formula CH2O) occurs:

CH2O(s) + O2(g) --> CO2(g) + H2O(g) ΔrH = -458 kJ/mol

How much energy (in kJ) is released when a 10.0-lb wood log burns completely?

Answer 1

one approach
find the molecular mass for 1 mole of wood in grams
find the equivalent amount of grams for 10 lbs of wood
set up the ratio let x be the unknown energy in kj lost

Answer 2

How do I know the mass for 1 mole of wood in grams?

Answer 3

how much energy is released, you can use formula
delta H = -q/ n

delta H ->enthalpy of combustion
q -> energy released
n -> moles of reactant

So, in this case you must find the moles of reactant first
Remember, you can find the moles using: gram/ Mr

Answer 4

I'm still confused, What is the Mr at the end? How do I find the moles?

Answer 5

Find how many molecule relative of CH2O?

Answer 6

Is it 30?
C=12
H2=2
O=16

Answer 7

yes

Answer 8

Ok, so 10 lb is 4535.92 grams.

4535.92/30=151.197?

Answer 9

yes

Answer 10

So,
delta H=-q/151.197?

Answer 11

yes, with q is 458

Answer 12

Ok, so

delta H= 458/151.197

delta H=3.029

Answer 13

no..

Answer 14

-458/151.197

Answer 15

Oh sorry, I thought that the negatives would cancel out.

-3.029

Answer 16

that's correct :)

Answer 17

Ok, thank you for walking me through the problem. :)

Answer 18

you're welcome :)