Answer

 Sodium hydroxide has a molecular weight of 40.
 NaOH
 Sodium Na 23
 Oxygen  0 16
 Hydrogen H  1
 Total = 40

 Therefore a 1 molar solution of sodium hydroxide contains 40g/L of NaOH.

 A 10% solution (w/v) would be 10g per 100ml or 100g per litre.
 A 40% solution (w/v) is therefore 400g /L

 So a 40% solution NaOH is 10 molar .