Question

HBr + KHCO3 --> H2O + KBr + CO2

Answer 1

16 grams.   
 Looking at the reaction equation, it's evident that for each mole of HBr
consumed, 1 mole of CO2 will be produced. So let's calculate the molar mass
of HBr and CO2 first. 
 Atomic weight hydrogen = 1.00794  
 Atomic weight bromine = 79.904  
 Atomic weight carbon = 12.0107  
 Atomic weight oxygen = 15.999   
 Molar mass HBr = 1.00794 + 79.904 = 80.91194 g/mol 
 Molar mass CO2 = 12.0107 + 2*15.999 = 44.0087 g/mol 
 Moles HBr = 30 g / 80.91194 g/mol = 0.370773461 mol 
 Grams CO2 = 0.370773461 mol * 44.0087 g/mol = 16.317258 g   
 Rounding to 2 significant digits gives 16 grams.