Lava Mobiles — internal storage of phone

I bought lava iris 412 mobile, when the time of buying seller said, phone having 512 mb of internal storage for apps or rom but after buying it was only 100mb, same thing i asked to the person who sold phone to me but he said i dont know. So that is why i am complaining you about this, the phone is unable to install even one app like whatss app, amazon, so pls feel as your responsible and solve this problem