Ammonia reacts with oxygen according to the equation: 4 nh3 (g) + 5 o2 (g) → 4 no(g) + 6 h2o(g) δhrxn= −906 kj calculate the heat (in kj) associated with the complete reaction of 155 g of nh3.

First, we need to get the number of moles of NH3:
no.of moles = mass of NH3 / molar mass
when we have the mass= 155g & the molar mass of NH3 = 14+(1x3) = 17 g/mol
∴ no.of moles = 155g/17mol/g
                        = 9.12 moles 
∴ the heat (in Kj) = no.of moles of NH3 * (ΔHrxn/4moles NH3)
                            = 9.12 moles * (-906 / 4mol NH3)
                            = -2065.68 KJ 

The answer is 1.82 j/g°c