1. Solid chromium(iii) oxidereacts with dihydrogen sulgide gas to form solid chr
ID: 850185 • Letter: 1
Question
1. Solid chromium(iii) oxidereacts with dihydrogen sulgide gas to form solid chromium(iii) sulfide and water.
a) Write the balanced chemical equation for the reaction.
b) Calculate the i) number of moles and ii) the mass of chromium(iii) oxide needed to produce 421 g of chromium (iii) sulfide.
c) look at your answers to part b) and calculate the mass of dihydrogen monosulfide needed to complete the reaction.
d) if 100 g of chromium (iii) reacts with 75g of dihydrogen monosulfide, calculate the theoretica yield of chromium (iii) sulfide. which reactan is LR, and which is in excess?
Explanation / Answer
the reaction is given by Cr2O3 + H2S ------> Cr2S3 + H2O .
a) for having three equivalents of S on the right hand side , we must have 3 equivalents of H2S on the left hand side. then we nee dto have 3 equivalents of H2O on the r.h.s to balance the 3 equivalents of H2S on the l.h.s .
the resulting balanced equation would be , Cr2O3 + 3 H2S -------> Cr2S3 + 3 H2O .
b)from the above equation , it is clear that , 1 mole of Cr2O3 produces 1 mole of Cr2S3 .
now , molar weight of Cr2O3 = 2 * ( 52 ) + 3 * ( 16 ) = 152 & molar weight of Cr2S3 = 2 * ( 52 ) + 3 * ( 32 ) = 200 .
which means 1mole or 152 gms of Cr2O3 produces 1mole or 200gms of Cr2S3 .
So, to produce 421 gms of Cr2S3 , which is equivalent to 2.105 moles .
i) we need 2.105 moles of Cr2O3 , as the relation is 1 : 1.
ii) 2.105 moles of Cr2O3 is equivalent to 2.105 * 152 = 319.96 gms . this the required answer
c) if one mole of Cr2O3 is used , we need 3 moles of H2S to complete the reaction .
As we earlier used 2.105 moles of Cr2O3 , we will need 2.105 * 3 = 6.315 moles of H2S .
molar weight of H2S is ( 2 *1 ) + 32 = 34 .
hence , we will need 6.315 * 34 = 214.71 gms of H2S
d) NOTE : you said "100 gm of Chromium(iii) reacts with " . i suppose your intention was " 100gm of Chromium(iii Oxide " because only Chromium doesnot react with H2S .
As 1 mole of Cr2O3 ( 152 gms ) requires 3 moles of H2S ( 102 gms ) , 100 gm of Cr2O3 will require
( 100 / 152 ) * 102 = 67.1 gm of H2S .
but as mentioned above , we were given , 75 gm of H2S .
which clearly specifies that the reactant H2S is in excess and the reactant Cr2O3 is LR .
have a good day .
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