The reactions are as follows:
(i) 2C2H6+2Br2→2C2H5Br+2HBr
(ii)2C2H5Br+2Na→C4H10+2NaBr
Reactions (i) and (ii) reveal mole ratio as:
C2H6:C2H5Br:C4H10::2:2:1
Now, moles of butane formed =5558.
Mole of C2H5Br actually required =2×5558×10085
(∵ reaction efficiency is85%)
Now, similarly mole of C2H6 actually required
=2×5558×10085×10090=2.479 mol.
(Since, this time effciency is 90%)
Therefore, volume of C2H6 at NTP actually required
22.4×2.479=55.53 L.
So, the answer is 56.