The treatment of C₃H₇OH with CH3MgI releases 2.24mL of gas at STP. The mass of
C₃H₇OH added is ______(in milligrams)

C₃H₇OH has only one active H- atom
So, 1 mol of C₃H₇OH gives one mole of CH₄ gas.
Mol . wt of C₃H₇OH = 60 gm
2240 mL of CH₄ is obtained from 60g of C₃H₇OH
2.24 mL of CH₄ is obtained from ____
$=\frac{60\times 2.24}{2240}=6\times {10}^{-3}\mathrm{gm}\left(\text{ or }\right)6\mathrm{mg}$