It is the independent variable because you can ‘manipulate’ or ‘change’ it.
First, we assume that helium behaves as an ideal gas such that the ideal gas law is applicable.
PV = nRT
where P is pressure, V is volume, n is number of moles, R is universal gas constant, and T is temperature. From the equation, if n, R, and T are constant, there is an inverse relationship between P and V. From the given choices, the container with the greatest pressure would be the 50 mL.
Answer : The heat released by the combustion of 1.121 mol of hexane (kJ) is, 
Explanation :
The given balanced reaction is,

Given:
Moles of hexane = 0.121 mol
= -8326 kJ/mol
The negative sign indicate that the reaction exothermic or amount of heat released.
As, 1 mole of hexane released heat = 8326 kJ
So, 0.121 mole of hexane released heat = 8326 × 0.121 = 1007.446 ≈ 
Therefore, the heat released by the combustion of 1.121 mol of hexane is
.
The answer is calving
hope that helped