Ok, lets see the definitions of percent yield, actual
yield and theoretical yield.
Percent yield is the ratio of actual yield to theoretical
yield.
Actual yield- amount of product produced in the EXPERIMENT.
Theoretical yield- max amount of product produced through CALCULATIONS
% yield= actual yield (from experiment)/theoretical yield
(from calculation) *100
1st Step: Write the reaction
H2 + Br2 --> 2 HBr
2nd Step: Get the mass ratio of H2 and HBr to find theoretical yield
1 mole H2 gives 2 moles HBr ( molar mass of H2= 2 g/mol, HBr= 81 g/mol)
2 g H2 gives 2*81= 162 g HBr
30 g H2 gives 162*30/2 = 2430 g HBr ( The equation is 2 g H2/ 30 gH2= 162 g HBr/ x g HBr)
So theoretically 2430 g HBr are produced by calculation ( THEORETICAL YIELD)
By experiment 85 g HBr are produced as it is given at the question ( ACTUAL YIELD)
% yield= actual yield / theoretical yield *100 = 85/2430 *100= 3.5 %
The percent yield is 3.5 %.