# A level chemistry uncertainty question

The balance used to weigh the 680 mg of iron wire has an uncertainty of ±0.005 g
A container was weighed and its mass was subtracted from the total mass of the
container and wire.
Calculate the percentage uncertainty in using the balance.

I calculated and got 0.74% but the answer is 1.47%! My mind is not working pls help)
The balance used to weigh the 680 mg of iron wire has an uncertainty of ±0.005 g
A container was weighed and its mass was subtracted from the total mass of the
container and wire.
Calculate the percentage uncertainty in using the balance.
I calculated and got 0.74% but the answer is 1.47%! My mind is not working pls help)

Two measurements have been taken as 680 mg is the difference between the mass of the empty container and the combined mass of the container and the iron.

Because two measurements have been taken, the uncertainty is doubled:

0.735…% x 2 = 1.47%
2 measurements have been taken so you do 0.005 x 2 = 0.01
since 0.005 uncertainty is in grams and the mass 680 is mg you need to divide 680 by 1000 to get 0.680g
then you do (0.01/0.680) x 100 = 1.47%