When 1 mol of methane is burned at constant pressure, −890 kJ/mol of energy is released as heat. If a 3.72 g sample of methane is burned at constant pressure, what will be the value of ∆H? (Hint: Convert the grams of methane to moles. Also make sure your answer has the correct sign for an exothermic process.) Answer in units of kJ.

-206.9kJ

Explanation:

Heat of combustion = -890kJ/mol

Mass of methane burnt = 3.72g

Unknown:

Heat liberated  = ?

Solution:

Heat of combustion is the heat liberated when one mole of an element or compound is burnt completely in excess oxygen gas.

    To solve this problem;

   Find the number of moles of methane;

             Number of moles  =

 Molar mass of CH₄   = 12 + 4(1) = 16g/mol

     Number of moles =   = 0.23moles

Now, heat liberated by 0.23mol of methane  =  -890kJ/mol x 0.23mol

                                                                          = -206.9kJ

they are all solids.