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From your laboratory work, you should know the common causes of error in a gravimetric analysis such as this. Not drying the precipitate completely is a common error: the precipitate is too heavy because it still contains a lot of water. As long as you add sufficient precipitating solution (barium nitrate in this case), it does not matter how much excess you add (within reason). The reaction involved here is an ionic one (a precipitation reaction) and these are generally fast so you do not need to wait long for the reaction to go to completion (though you do need to stir for several minutes to ensure that all the barium sulfate coagulates into an easily filterable precipitate.