A sample of hydrated sodium phosphate has a mass of 190 g. Of this, 108 g is the mass of water of crystalisation. What is the molar mass of the hydrated sodium phosphate ?

Dear Student

The molar mass of $N{a}_{3}P{O}_4$ is 164 g. 
Now as the given sample of weight 190 g contains 108 g of water or  6 molecule of hydrated water. 
The remaining weight of the sample is the weight of the sodium phosphate = 190 - 108 = 82 g 

This infers that the sample contain just half of molar weight of the $N{a}_{3}P{O}_4$. 
So, the molar mass of the hydrated sodium phosphate will be 480 g. Which contains 12 molecule of hydrated water. 
The formula of the hydrated sodium phosphate is $N{a}_{3}P{O}_4.12 H_{2}O$ .

Regards