Amongst the following compounds
(I) H₅P₃O₁₀ (II) H₆P₄O₁₃ (III) H₅P₅O₁₅ (IV) H₇P₅O₁₆
non-cyclic phosphates are:

Conditions for closed cyclic stmcture -
 (i) Number of central atoms in given covalent compound should be greater than or equal to 3.
(ii) Individual surrounding atom should be integral multiple of number with central atoms.
Non-cyclic phosphates are ${\mathrm{H}}_5{\mathrm{P}}_3{\mathrm{O}}_{10},{\mathrm{H}}_6{\mathrm{P}}_4{\mathrm{O}}_{13}\text{ and }{\mathrm{H}}_7{\mathrm{P}}_5{\mathrm{O}}_{16}$.