If it was still in her possession then I would have gifted it to her. Now that it is in yours you can't prove anything so now what to do.
Be honest and take the emotion out of it. Do you really think she is unhinged and vindictive enough to knowingly mix powder to cause you injury or death? If so, dump it and move on. If your not sure or don't think so, inspect it carefully then if it looks good try some known minimum loads and see what happens.
I have had some crazy ex's but none that I think would try and kill me in an explosion although one did loosen lug nuts causing a wheel to fall off.
A few pounds of powder is not much in the grand scheme of things. Ease your mind by throwing it out and move on is what I'd do.