Two solids A and B having molar masses 200 and 300 react to form another solid C as shown

$2A\left(s\right)+3B\left(s\right)\to C\left(s\right)$

If $\mathrm{\Delta }{H}_{\text{combustion }}^{\circ }$ of A, B and C are -200, -300 and -400kj respectively then maximum heat which can be released at constant pressure if total 2600 g of A and B is taken, is given by :

In the reaction we have,

$\begin{array}{l}2A\left(s\right)+3B\left(s\right)⟶C\left(s\right)\\ \begin{array}{rl}\mathrm{\Delta }H& =2\mathrm{\Delta }{H}_{\text{combustion of }A\left(s\right)}+3\mathrm{\Delta }{H}_{\text{combustion of }B\left(s\right)}\\ & -\mathrm{\Delta }{H}_{\text{combustion of }C\left(s\right)}\\ & =2(-200)+3(-300)-(-400)\\ & =-900\mathrm{kJ}\end{array}\end{array}$

Maximum heat will be released when A and B are taken in mole ratio as ratio of their stoichiometric coefficient (i.e., 2 : 3 Let moles of A be 2x and moles of B be 3x

$\begin{array}{l}2x\times 200+3x\times 300=2600\mathrm{g}\\ x=2\end{array}$

Moles of A taken = 4; Moles of B taken = 6 Heat released = 900 x 2 = - 1800 kJ (900 kJ is released when 2 mole of A react with 3 mole of B)