Volumes don't relate into weights exactly. A spoonful of water won't weigh the same as a spoonful of powder or a spoonful of cereal.
However, as a rough guide, water is 5g per teaspoon, so about 0.17 oz. The powder is likely to be similar, probably a bit less. If you can't find an accurate measurement of that anywhere, then use 5g for now.
It's going to depend on the form of the sweetener. The sweetener itself is so light that it's not really measureable without a chemistry lab; what you would really be weighing is the starchy powder that they mix it with so you can see it. The stuff in packets has less of that per drop of sweetener than the stuff in boxes or bags, and every brand is a little bit different, too.
If you're using a teaspoon at a time, it's probably not worth the effort to weigh and measure. It's only going to have a fraction of a gram of carbohydrate (from the powder) and about 2 calories. If you start using it to bake or to mix large amounts of sweetened drinks or something, then you can find the information for a cup by googling, and make a custom food. (It might even be in the database already as a shared entry.)
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