The chemical equation is missbalaced.
The right balanced chemical equation is:
2NaHCO3(s) + 129 kJ ---> Na2CO3(s) + H2O(g) + CO2(g)
Then, now you know that 2 moles of NaHCO3 requires 129 kJ heat, and you just need to convert 25.5 grams of NaHCO3 into number of moles to calculare the amount of heat needed.
A) Number of moles in 25.5 g of NaHCO3, n:
n = grams / molar mass
molar mass NaHCO3 = 23 g/mol + 1g/mol + 12g/mol + 3*16g/mol = 84 g/mol
n = 25.5 g / 84 g/mol = 0.304 mol
B) Heat required
Make the proportion with the theoretical ratio:
2mol / 129 kJ = 0.304 mol / x
=> x = 0.304 mol * 129 kJ / 2 mol = 19.608 kJ
Answer: 19.6 kJ
