It was raining. and I was walking on a road when a car and splashed dirty water on my pants. The part over which water split is from my toe up to knee. My pants were fully wet as a result. The water on the road may contain urine and other najasat. Besides, someone may have urinated beside the road. But I didn't change my pants. I performed my salat with that pant. Will my salat be valid?
Well, in my opinion it depends upon your first assumption regarding the water. If for example the water is rain water on a regular street with no obvious source of najasah, then your fist assumption is that the water is not-najis and you act upon it unless you discover otherwise.
It is not enough to have a suspicion of some rare event (e.g. urination) to have occurred to consider that the water is najis. You need to witness it or have two trustworthy persons inform you.
Off-course if the najis-causing event is frequent in that place, then you would assume the water to be najis.
Also, if the rain-water is plentiful, then I would consider it to be not-najis even in events where a small amount of najasah be present. For example, if one sees another urinate in an otherwise not-najis plentiful stream of water, then the water is considered not-najis. (i.e. urination in a river does not cause the river to become najis)
This is my personal understanding from the shia sect.
An old question but anyway, in case it may help someone in the future, the answer is "yes, you prayer is valid", at least according to Shiite jurisprudence ...
Indeed, as ageneral rule, everything is clean unless you are certain that it is Najis ... As a critical example, assume there is a carpet in the room and you have seen a droplet of some najis thing has fallen over the carpet, but you don't know exactly where, then every single dot on the carpet is clean for you as you are not sure if the najis droplet has fallen exactly over there. Only if you are going to use the whole surface of the carpet then that would be problematic, as you certainly know somewhere on the carpet there has been fallen a Najis droplet ...