<u>Answer:</u> The mass of methanol that must be burned is 24.34 grams
<u>Explanation:</u>
We are given:
Amount of heat produced = 581 kJ
For the given chemical equation:

By Stoichiometry of the reaction:
When 764 kJ of heat is produced, the amount of methanol reacted is 1 mole
So, when 581 kJ of heat will be produced, the amount of methanol reacted will be = 
To calculate mass for given number of moles, we use the equation:

Moles of methanol = 0.7605 moles
Molar mass of methanol = 32 g/mol
Putting values in above equation, we get:

Hence, the mass of methanol that must be burned is 24.34 grams