Molality of 1M NaOH solution is (density of NaOH solution = 1.04 g/cc)

$n_{NaOH}=1$

$V_{solution}=1000ml$

$d_{solution}=1.04\text{ g/}ml$

$w_{solution}=1.04\left(1000\right)=1040g$

$w_{solvent}=\left(1040-40\right)=1000g$

 $\therefore$ molality =  $1\times \frac{1000}{1000}$