Can someone explain on how they derived to the answer of 20.0L?
Work out the number of moles of chlorine gas that yields 75 g of the product, then use the molar volume at STP, 22.4 litres/mol.
How many moles of 1,1,2,2-tetrachloroethane is formed? Molar mass is 167.8 g/mol, so you have formed 75 / 167.8 = 0.4469 mol.
The equation tells you that 1 mole of 1,1,2,2-tetrachloroethane is formed for ever 2 moles of chlorine gas. Therefore if you formed 0.4469 mol of 1,1,2,2-tetrachlorothane, you would have needed 0.8938 moles of chlorine.
If you assume that 1 mole of a gas occupies 22.4 litres at STP, then 0.8938 moles will occupy 0.8938 x 22.4 = 20.021 litres. To 1 decimal place, that is 20.0 litre.