This should help :)
Example 1: A 36.0 g sample of water is initially at 10.0 °C. How much energy is required to turn it into steam at 200.0 °C? (This example starts with a temperature change, then a phase change followed by another temperature change.)
Solution:
Answer: P2O5
Explanation: You divide both subscripts by 2 to get the empirical formula.