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To find the molecular mass, we need to add the molar mass of each elements
Hydrogen has a mass of 1
Sodium has a mass of 23
Oxygen has a mass of 16

The number of element Na in NaOH is 1, O is 1, and H is 1.

Add their mass:
Na :23x1=23
O : 16x1=16
H :   1x1=1

Therefore the molecular mass of NaOH is 49