I pray that every single soul in the world will be saved. Is this praying for the impossible? Not really. To easily see if something is right or wrong, just look at what would happen if everyone does it. In the same way, if everyone would pray for the salvation of all souls, the prayer is clearly answered. So it's not impossible, only improbable. Nevertheless, the faithful should not hesitate to pray for what's not likely to happen, as nothing is impossible (not improbable) for God.