1) State the balanced chemical equation
Na3 PO4 + 3 AgNO3 → 3NaNO3 + Ag3 PO4
sodium phosphate silver nitrate sodium nitrate silver phosphate
2) State the molar ratios
1 mol Na3PO4 : 3 mol AgNO3 : 3 mol NaNO3 : 1 mol Ag3 PO4
3) As you see 3 moles of Silver Nitrate react with 1 mol of Na3 PO4, then you will need.
The you need to use the molar ratio 1:3 to calculate the number of moles of sodium phosphate
1 mol Ag NO3 * [ 1 mol Na3 PO4 / 3 mol Ag NO3] = 0.33 mol Na3 PO4
Answer: 0.33 mol sodium phosphate