Question

4 Ag + 2 H2S + O2 = 2 Ag2S + 2 H2O

a) What mass if silver sulfide is formed from the reaction of 0.120g of silver?
b) What mass of hydrogen sulfide is needed in the same process?

So I understand a, and I found the answer to be 0.138g. For b the text book says the answer is 0.0190g. I don't understand how they got to that answer because they never tell us the mass of the hydrogen sulfide

I'd really appreciate your help, thank you (:

Answer 1

Convert given data to moles, relate to the reaction ratios, then convert back to grams.

a) for 0.12g Ag = 0.0011mole Ag => produces 1/2(0.0011mole) Ag2S => (0.00056mole)(248g/mole) = 0.138g Ag2S

b) assume an excess of H2S is present, then the rxn will require at least 1/2(moles Ag used). From (a) moles Ag = 0.0011mole then => moles H2S required is 1/2(0.0011)mole = 0.00056mole H2S = (0.00056mole)(34g/mole)H2S = 0.019g H2S