To determine the molecules of butane we can either use multiple steps or use a combined equation based on the following formulas:
Molecules of butane = Moles × Avogadro's Number
Moles of Butane = Mass ÷ Molar Mass
Mass of Butane = volume × Density
∴ Molecules of butane
= [(volume × Density) ÷ Molar Mass] × Avogadro's Number
= ([(25.0 ml × 0.6011 g/ml) ÷ 58.12 g/mol] × (6.022 × 10²³ molecules/mol))
= 1.557 × 10²³ molecules