At STP (Standard Temperature and Pressure), 1 mole of any ideal gas occupies 22.4 L. Therefore, to find the number of moles in 5.60 L of Cl2 at STP, we can use the following formula:

Number of moles = Volume of gas (in L) / Volume occupied by 1 mole of gas at STP (22.4 L)

Number of moles = 5.60 L / 22.4 L/mol

Number of moles ≈ 0.25 moles

Therefore, there are approximately 0.25 moles in 5.60 L of Cl2 at STP.