Answer: 7.52*10^23 molecules.
Explanation: This is a classic Stoichiometry problem.
In one mole of any substance, there are 6.02*10^23 molecules. This number is called Avogadro's number. We are given 55 grams of Co2 so to convert that to moles, we divided by the molar mass of Co2. We find the molar mass by adding the molar masses of the elements that make up the compound.
There is one molecule of Carbon and two molecules of Oxygen in one molecule of Co2. From the periodic table, the molar mass of Carbon is 12.01 and 16.00 for Oxygen. 1(12.01)+2(16.00) gives us the molar mass. We then divided 55 grams by that mass to find the number of moles. We then multiply the number of moles by Avogadro's number (6.02*10^23) to find the total number of molecules.
You can use this method for solving any problem that asks you to find the number of atoms or molecules of some number of grams of a substance.