How to calculate the net charge of a polypeptide chain

I have spent a lot of time trying to understand this but can not. so please kindly help. if you a given a set of PH values and ask to find the net charges on a polypeptide chain. how do you do this. I understand that when the PH < PKa = Protonated and when PH > PKa = Deprotonated, and when PH = PKa = 50/50 change. so I was given a set of PH (e.g.) PH 3, 7, 10. how can I calculate the net charges. Please help.