In order for a trust to exist the three certainties must be satisfied - intention, subject matter and objects. You should go through each of these in relation to any problem style question. If any of the certainties aren't met the trust will fail.
If the intention isn't satisfied because of the use of precatory wording (E.g. Expressing a wish or desire that money be used for a specific purpose rather than stipulating exactly) the trust will fail but it may still be a valid gift meaning that the donee has no legal obligation to carry out the wishes of the doner.