Are prayers performed by a worshiper without ghusl from janabah, due to ignorance on his part, obligatory to be repeated?

You are obligated to make up the prayers you performed while in a state of janabah (definition) (major ritual impurity). If you know their number, make them up. If you do not know, continue making them up until you are reasonably certain that you have fulfilled your obligation. This is the view of the majority of scholars. However, Shaykh al-Islam Ibn Taymiyyah and those who agreed with him held that making them up is not obligatory. A Muslim must learn the individual obligations such as purity and prayer.