Consider the following balanced thermochemical equation, sometimes used for H2S production.?

Question

1/8 S8(s) + H2(g) H2S(g) Hrxn = -20.2 kJ

(c) What is Hrxn when 3.4 mol S8 reacts?
(d) What is Hrxn when 20.0 g S8 reacts?

Answer 1

Hrxn given is the value for the reaction as written, with 0.125mol of S8. So, for 3.4mol of S8, I'd have Hrxn of the one given times 27.2 (27.2= (8*3.4) you'd have to multiply the coefficient on S8 by this value to get 3.4S8's). 27.2*-20.2= -549.44kJ

For 20g of S8, you need to know how many moles that is. Molar mass of S8=8(32.06g/mol) from periodic table, which equals 256.48g/mol. Then divide mass by molar mass to get number of moles. 20/256.48=0.078mol S8. You need to know what to multiply the equation, and hence the given Hrxn by. So divide 0.078/0.125=0.624. 0.624*-20.2= -12.6kJ