Okay, I made quite a few assumptions here:

Assume that all the chlorine reacted with the phosphorus.

And for the PCl

_{3}, PCl

_{5} formed, It fully hydrolyse to H

_{3}PO

_{3} and H

_{3}PO

_{4}.

So

PCl

_{3} + H

_{2}O

H

_{3}PO

_{3} +3HCl (pKa = 1.3, 6.7)

PCl

_{5} + H

_{2}O

H

_{3}PO

_{4} +5HCl (pKa = 2.15, 7.20, 12.3)

Looking at the pKa values, in methyl orange, the phosphurous species present will be H2PO

_{4}^{-} and H2PO

_{3}^{-} predominantly.

In pheno, the phosphurous species present will be HPO

_{4}^{2-} and HPO

_{3}^{2-} predominantly.

So in the 10ml reacted, let there be x mol H

_{3}PO

_{3} and y mol H

_{3}PO

_{4}4x + 6y = 0.0071 x 0.25

5x + 7y = 0.0087 x 0.25

solving, x= 3.125 x 10

^{-4}, y= 8.75 x 10

^{-5}thus in the mixture [itex]\frac{PCl_3}{PCl_5}= \frac{25}{7} [/itex]

since 0.32258 mol of P reacted,

nPCl

_{3} = 0.2520156 mol

nPCl

_{5} = 0.070565 mol

so 39.365g of Cl

_{2} was used.

Didn't check, so might have math error somewhere.