Given reactions are
(i) 2C2H6+2Br2→2C2H5Br+2HBr
(ii)2C2H5Br+2Na→C4H10+2NaBr
Reactions (i) and (ii) reveal mole ratio
C2H6:C2H5Br:C4H10::2:2:1
Now mole of butane formed = 55/58
Mole of C2H5Br actually required = 2×5558×10085
(∵ reaction efficiency is 85%)
Now similarly mole of C2H6 actually required
= 2×5558×10085×10090=2.479mol
(Since, this time effciency is 90%)
Therefore, volume of C2H6 at NTP actually required
22.4×2.479litre=55.53litre
So, answer is 56.