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this detail throws it off
test for 0.50 kg instead

light the bags on fire, you didn't say I would be fired for vandalism or negligence

Missed the fact you can only weight it once. There is no 100% guaranteed way to do it with only one weighing because there are certain cases where it will take the full ceil(log(10)) -1 = 3 measures to figure it out. Here is how to maximize your chances.

let x be an integer between 0 and 5.
let P(x) be the probability of guessing right.
P(x) = ((5-x)/5 * 1/(10-2x)) + (x/5 * 1/x), 0 < x < 5
P(x) = (5-x)/(5*(10-2x)) + 1/5, 0 < x < 5
P(x) = (15 - 3x)/(50 - 10x), 0 < x < 5
P(x) = 3/10, 0 < x < 5

For the whole domain of x we have a piecewise function.
P(x) = 1/10, x=0 | 3/10, 0 < x < 5 | 1/5, x=5
To maximize P(x), x can be 2, 3, or 4.

1. Put x bags on either side of the balance and weigh them. If equal, go to 2a. Else, go to 2b.
2a. Guess at random from the bags you did not weigh.
2b. Guess at random from the lighter side of the balance.
This strategy gives you a 30% success rate.
The reason why we have we don't test the situations with unequal amount of bags is that it gives us no new information.