First of all, we must know the Avogadro's number:
1 mol of CO2 = 44.0 g (from the molar mass) = 6.02x10^23 molecules of CO2
How must we proceed?
44.0 g CO2 ------------6.02x10^23 molecules CO2
2.59 g CO2 ------------ x
x = 3.54x10^22 molecules CO2
Answer: 3.54x10^22 molecules CO2
