Answer
See the answer below.
Work Step by Step
The number of moles of Na2CO3:
$0.200\ m=n/(125/1000\ kg)\rightarrow n=0.025\ mol$
The mass of Na2CO3 required:
$0.025\ mol\times 105.99\ g/mol=2.65\ g$
The number of moles of water:
$125\ g\div18.015\ g/mol=6.94\ mol$
Mole fraction of Na2CO3:
$0.025\div(0.025+6.94)=0.0036$
