Urgent chem question??? HELP!???

we are learning about the prefix system for naming covalent compounds. we are also learning about finding the formulas from covalent compound names. But there is a problem. my teacher says that he will not put a prefix in front of the first element in a covalent compound. so for instance, phoshorus pentasulphide we would ume is PS5 but its P2S5. How can we find the correct formula if there is no prefix in front of the first element?