3.49g of ammonia at STP occupies a volume of 4.48 ${dm}^3$ calculate the molar mass of ammonia.

The correct option is C 17.45 $g/mol$
Volume = 4.48 $d{m}^3$ = 4.48 L

Gram Molecular Mass = 22.4 L at STP

So,

4.48 L of ammonia gas weighs = 3.49 g

22.4 L of ammonia weighs = $\frac{22.4\times 3.49}{4.48}$

= 17. 45 g / mol