How much carbon dioxide has to be bubbled into aqueous sodium hydroxide to obtain a solution containing 3.5 mol of sodium hydrogen carbonate?

Answer 1

CO2 + NaOH ==> NaHCO3
You want 3.5 mol NaHCO3 so you must bubble 3.5 mol CO2 (and the question doesn't ask but you must also have at least 3.5 mol NaOH present)