Suppose that a solution containing 3.50 grams of sodium phosphate (na3po4) is mixed with a solution containing 6.40 grams of barium nitrate (ba(no3)2). calculate the theoretical yield of barium phosphate (ba3(po4)2) during this reaction.
2na3po4 + 3ba(no3)2 ba3(po4)2 + 6nano3

We are given 
3.5 g Na3PO4
6.4 g Ba(NO3)2

and the reaction
2 Na3PO4 + 3 Ba(NO3)2 > Ba3(PO4)2 + 6 NaNO3

Convert the masses to moles using molar masses of N3PO4 and Ba(NO3)2
Next,  determine the limiting reactant by using stoichiometry:
moles Na3PO4 (1 / 2) = moles Ba3PO4
moles Ba(NO3)2 (1 /3)  = moles Ba3PO4

The lower amount of moles will be maximum and the reactant is limiting
Convert moles to mass and divide it by the mass of the limiting reactant to get the yield.