Suppose I wear clothes having dried urine on them then after some time I change the clothes and wear clean clothes and do Wudu. Now, will I be considered pure? If not then what do I have to do? And if I am not considered pure will I have to pray the Salat that I prayed in that state again Qada?
You would be considered as a pure person if you are not sure that the dried urine definitely has toughed your body (by things like water...). Of course as a significant ruling, saying prayer by Najis (impure) clothes is not true/accepted. For more precaution, you'd better wash your clothes and go to bathroom and wash yourself to be certain regarding this issue (to be clean/pure).