Question

Suppose that coal of density 1.5 g/cm3 is pure carbon. (It is, in fact, much more complicated, but this is a reasonable first approximation.) The combustion of carbon is described by the equation C(s) + O2(g) −! CO2(g)
H◦ = −394 kJ
What is the value of q (heat) when a lump of coal of size 8 cm×7.5 cm×7 cm is burned?
Answer in units of kJ

The next question follows the first one
:
What mass of water can be heated from 27◦C to 100◦C by burning this piece of coal?
Answer in units of g

Answer 1

394 kJ x (630/12) = ?

q = mass H2O x specific heat H2O x (Tfinal-Tinitial)
Substitute and solve for mass H2O. Remember to use q in J not kJ.

Answer 2

For the first question, the answer I got was -20665.3 kJ and it was correct. However, every time I plug that answer into the equation I get the wrong answer for the second question. I've converted the aforementioned number into joules but I still got the wrong answer when I plugged it into the equation. I'm not sure what I'm doing wrong.

Answer 3

Using 12.011 for atomic mass C I obtained 20,666,000 J.
1. Did you substitute -20665.3 kJ convert to J. If so that's the problem. That is a + number when you heat the water.

2. If you used a + number then count the number of significant figures. That's often a problem with these on-line databases.

Let me know please and we can try something else if this doesn't fix the problem.