Does the law of definite proportions apply to mixtures?
Think of this from the perspective of Rocky Road Ice Cream, which is a mixture because the nuts and marshmallows are not homogenous and can be removed by a no-chemical means.
If the Law of definite proportions held true for mixtures, every container of Rocky Road would be uniform in the number of marshmallows, the number of nuts and the uniformity of their distribution throughout the ice cream. Each bite you would enjoy would be exactly the same.
I hope this was helpful.