To calculate the moles of CO2 formed, we need to consider the reaction that takes place between HCl and Na2CO3:

2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

According to the balanced chemical equation, 2 moles of HCl react with 1 mole of Na2CO3 to produce 1 mole of CO2.

First, we need to convert the volume of HCl solution from milliliters (mL) to liters (L):

15.5 mL HCl solution = 15.5 / 1000 L = 0.0155 L

Next, we can calculate the number of moles of HCl in the solution using its concentration (3.00 M):

Moles of HCl = Concentration × Volume

Moles of HCl = 3.00 M × 0.0155 L = 0.0465 moles

Since 2 moles of HCl are required to react with 1 mole of Na2CO3, we can determine the moles of CO2 formed using stoichiometry:

Moles of CO2 = Moles of HCl / 2

Moles of CO2 = 0.0465 moles HCl / 2 = 0.02325 moles

Therefore, when 15.5 mL of 3.00 M HCl solution reacts, 0.02325 moles of CO2 form.