The density of 3M solution of NaCl is 1.25 g mL^-1. Calculate the molality of the solution.

$M=3mol{L}^{-1}$
Mass of NaCl
In 1 L solution = 3 x 58.5 = 175.5 g
Mass of 
1L solution = 1000 x 1.25 =1250 g
(since density  = 1.25 g mL^-1)
Mass of water in solution = 1250 – 75.5
 = 1074.5g
$Molality=\frac{No.ofmolesofsolute}{Massofsolventinkg}$
$=\frac{3mol}{1.0745kg}=2.79m$
OR 
$m=\frac{M\times 1000}{\left(d_{solution}\times 1000\right)-M\times GMMofsolute}$