The definition of concentration is
C=nV
where concentration is is moles per litre (M), n is the number of moles (mol) and V is the volume in litres (L). Make sure to use these units to get the right answer i.e.,
C=0.35 M
L=275.0⋅10−3 L
Now rearrange the concentration formula to solve for the number of moles of Na3PO4. I will round my answers to two significant figures , as that is the least amount given in the question.
n(Na3PO4)=C⋅V=0.35⋅275.0⋅10−3=9.6⋅10−2 mol
The number of moles is a measure of how many particles there are. The chemical formula shows that for every Na3PO4 salt particle, there are three Na+ ions, so we need to multiply the answer by three, giving
n(Na+)=3⋅n(Na3PO4)=3⋅9.6⋅10−2=0.29 mol