Calculating enthelpies of combustion from a bomb calorimeter?

[crimsonn]

Homework Statement

A sample of K (s) of mass 2.740g undergoes combustion in a constant volume calorimeter at 298.15K . The calorimeter constant is 1849 J/K , and the measured temperature rise in the inner water bath containing 1450 of water is 1.60K
Calculate the enthalpy of formation for K2O

Homework Equations

ΔH(combustion) = C(cal)ΔT +nC(H2O)ΔT

n= # of moles

The Attempt at a Solution

I pretty much know already how to do this problem and I've worked it out fully (I figured out the enthalpy of combustion and then divided by the number of moles of K I had), but I realized my answer was off by a factor of two.

Is this because the balanced equation is 2K + (1/2) O2 ----> K2O
and it takes two moles of potassium to make a mole of K2O?

I'm just unsure of when I should have taken the factor of two into account (assuming that my reasoning is correct)

 

[nate808]

it is important to always double check your work and make sure your calculations are accurate. In this case, your reasoning is correct. The balanced equation does show that it takes two moles of potassium to make one mole of K2O. Therefore, when calculating the enthalpy of formation for K2O, you should have divided the enthalpy of combustion by two, not the number of moles of K.

To avoid making this mistake in the future, always make sure to carefully examine the balanced equation and take into account any coefficients that may affect your calculations. It is also helpful to double check your work and make sure all units are consistent throughout the calculation.