Question

An error of 0.5 oC in reading the temperature drop of the metal is what percentage error?

Answer 1

Percentage error = [(measured value - accepted value)/accepted value]*100%.

You have a0.5 degree error for each measurement - so the % error is (0.5/temp)*100 - these errors are additive if you have a start and finish temp so a 4% error at start and a 2% error at finish gives a 6% error in your tempp change estimate.

I think anyway maybe someone would confirm that for you...

The 12 times is an estimate of your precision, or reliability - your estimate of Standard deviation of the 12 tells you how reliable the data were, whereas the error of the thermometer informs about accuracy of the measured change.