How many kilojoules are absorbed when 280 g of pcl5 reacts? pcl5(g)→pcl3(g)+cl2(g)δh=+67kj?

The molar mass of PCl5 is 208.24 g/mol. So calculating for the number of moles:

moles PCl5 = 280 / 208.24

moles PCl5 = 1.34 mol

From the equation, 67 kJ of heat is absorbed for every 1 mole hence:

total heat = 67 kJ / mol * 1.34 mol

total heat = 90 kJ

the millers tale is the answer.

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !