Question on Molecular Formula

If the molecular formula for calcium pyrophosphate is Ca₂P₂O₇, which one of the following (a) Na₂P₂O₇ or (b) Na₄P₂O₇, will the molecular formula for sodium pyrophosphate be?

Answer:

Pyrophosphate ion (P₂O₇^4-) has four negative charges which are satisfied by two calcium ions (Ca²⁺). So, the molecular formula of sodium pyrophosphate will be Na₄P₂O₇.