Question

# When 1g of anhydrous oxalic acid is burnt at 25$\circ$C, the amount of heat liberated is 2.835 kJ. $\Delta$H combustion is: (oxalic acid: C$_2$H$_2$O$_4$): A.- -255.15 kJB.- -445.65 kJC.- -295.24 kJD.- -155.16 kJ

Hint: With the help of given data, we have to calculate the heat combusted by heating the oxalic acid. So, calculate the moles of oxalic acid, and with the help of the heat liberated, and moles, the required value can be attained.

Now, first we will define the heat liberated, and the heat combustion.
We can say that when a chemical reaction takes place, and the amount of heat is absorbed, or we can say an endothermic reaction takes place.
Now, the other term is the heat of combustion; from the name itself we can say heat released on combustion, or heating the substance.
We have to calculate the amount of heat of combustion.
First, we will calculate the moles of oxalic acid. To calculate the moles of oxalic acid, let us know the molar mass of oxalic acid.
So, the molar mass of oxalic acid (C$_2$H$_2$O$_4$), it can be written as:
2(12) + 2(1) + 4(16) = 90
According to the question, 1 g of oxalic acid is used; so number of moles can be written as:
Number of moles = 1/90;
So, now we will calculate the heat of combustion by dividing the amount of heat liberated to that of the number of moles.
Thus, we can write it as:
$\Delta$H$_{comb}$ = -$\dfrac{heat liberated}{number of moles}$
So, now by substituting the values we get;
$\Delta$H$_{comb}$ = -$\dfrac{-2.835kJ}{1/90}$,
$\Delta$H$_{comb}$ = -255.15 kJ
Thus, in the end we can conclude that the heat of combustion for oxalic acid is -255.15 kJ.

Hence, the correct option is (A).

Note:
Don’t get confused why we put up the negative sign before the value of heat of combustion then we know that the energy is released in the process; it is an exothermic reaction. So, we have put up the negative sign.