Hbr + khco3 --> h2o + kbr + co2 if i do this reaction with 30 grams of hbr, how many grams of carbon dioxide will be formed?

The Bromium acid used in this is 30 grams. First, we need to convert it into mol so we can find out how much CO2 produced in mol unit.
The molecular weight for HBr would be 80.91g/mol, so the amount of HBr in mol would be: 30 gram/(80.91g/mol)= 0.37 mol

From the equation, you can conclude that for every 1 mol of HBr can produce 1 mol of CO2. The molecular weight for CO2 would be:12+ 16*2= 44g/mol. 
The amount of CO2 produced in grams would be: 1/1 * 0.37 mol* (44g/mol)= 16.31 grams