Here's how to calculate the number of CO₂ molecules in 51.5 g of CO₂:

1. Find the molar mass of CO₂

* Carbon (C) has a molar mass of 12.01 g/mol

* Oxygen (O) has a molar mass of 16.00 g/mol

* CO₂ has a molar mass of 12.01 + (16.00 x 2) = 44.01 g/mol

2. Calculate the number of moles of CO₂

* Divide the mass of CO₂ by its molar mass:

51.5 g CO₂ / 44.01 g/mol CO₂ = 1.170 mol CO₂

3. Use Avogadro's number to find the number of molecules

* Avogadro's number is 6.022 x 10²³ molecules/mol

* Multiply the number of moles by Avogadro's number:

1.170 mol CO₂ * 6.022 x 10²³ molecules/mol = 7.05 x 10²³ molecules of CO₂

Therefore, there are approximately 7.05 x 10²³ molecules of CO₂ present in 51.5 g of CO₂.