Which has the largest number of molecules?

To compare the number of molecules for each substance, we'll first need to calculate the number of moles of each substance. Then, using Avogadro's constant ($6.022\times {10}^{23}$ molecules per mole), we can determine the number of molecules in each sample.

•  N₂O₅

Number of moles of N₂O₅ = 54 g / 108.01 g/mol ≈ 0.5 moles

Number of molecules of N₂O₅ = Number of moles * Avogadro's constant
Number of molecules of N₂O₅ ≈ 0.5 moles * (6.022 x 10²³ molecules/mol)

 ≈ 3.011 x 10²³ molecules

•  CO₂

Number of moles of CO₂ = 28 g / 44.01 g/mol ≈ 0.636 moles

Number of molecules of CO2 = Number of moles * Avogadro's constant
Number of molecules of CO2 ≈ 0.636 moles * (6.022 x 10²³ molecules/mol)

≈ 3.834 x 10²³ molecules

•  H₂O

Number of moles of  H₂O = 36 g / 18.02 g/mol ≈ 1.997 moles

Number of molecules of  H₂O = Number of moles * Avogadro's constant
Number of molecules of  H₂O ≈ 1.997 moles * (6.022 x 10²³ molecules/mol)

 ≈ 1.202 x 10²⁴ molecules

• C₂H₅OH

Number of moles of C₂H₅OH = 46 g / 46.08 g/mol ≈ 0.998 moles

Number of molecules of C₂H₅OH = Number of moles * Avogadro's constant
Number of molecules of C₂H₅OH ≈ 0.998 moles * (6.022 x 10²³ molecules/mol) 

≈ 6.012 x 10²³ molecules

Comparing the number of molecules:

- 54 g of N₂O₅ ≈ 3.011 x 10²³ molecules
- 28 g of CO₂ ≈ 3.834 x 10²³ molecules
- 36 g of H₂O ≈ 1.202 x 10²⁴ molecules
- 46 g of C₂H₅OH ≈ 6.012 x 10²³ molecules

Hence, 36 g of H₂O has the largest number of molecules among the given substances.