Co-precipitation is the process by which undesirable materials are "carried down" and trapped on the suface or within the desired precitate. The precipitate is washed with distilled water to remove the co-precipitates which are more soluble that the precipitate. Care must be taken not to over-wash as some of the desired precipitate may be removed as well. Indicate, along with a brief explanation, whether the experimentally determined mass of analyte would be too high or too low if co-precipitation occurred, but the sample was not washed.
First I want to take issue with the question (yes, I know, you just copied it here). Where the problem states that co-precipitation is the process in which undesirable materials are "carried down" or trapped on the surface or within the desired precipitate. I want to point out here that if you are trying to prepare a very pure ppt then you don't want co-precipitation, or if such occurs, you need a way to clean it up. HOWEVER, in some cases, we WANT co-pptn to occur because the ppt may not be insoluble enough and/or we have such a small amount of material that it is completely soluble. Many years ago I spent six years developing a procedure in which the whole idea was to get as much co-pptn to occur as possible, even to the extent that we added material that would make a bulky ppt and would tend to trap anything that swam by. It worked, too.
Now for your question which I will answer with a question. If you have co-pptn, EXTRA material will be carried down, you DON'T wash it, so won't it contain too much material because of the extra contaminants co-pptd? And that will make the results what? Too high? or too low?