Error propagation when dividing by exact number

[NihalRi]

Homework Statement

Some Background - We are calculating the amount of acetylsalicylic acid in a sample using spectrophotometry. We were told to make sure to include the error in our answer. So first to calculate the moles of acetylsalicylic acid in a measured mass.

0.1620 ± 0.0005g measured using a digital balance
180.2gmol^-1 (molar mass of acetylsalicylic acid given in book)

2. Homework Equations

Mass/molar mass = moles

3. The Attempt at a Solution

0.1620 ± 0.0005g/180.2gmol^-1
I know that we divide the number and error by the fixed value but the error has way too many numbers so I am not sure how much to keep. My first guess is one more decimal place than the value but this does not make sense to me because is seems like there are too many significant digits in the answer for such a large error.

8.990±0.0277×10^-4mol

Calculating errors has always been a nightmare for me especially when the numbers are not pretty, I'd like to get this thing down so I would greatly appreciate your help.

 

[epenguin]

My reaction is to think you need to know what the error of your spectrophotometric measurement is . and my guess is that this is so sufficiently more than the 0.03% is that right? weight error range that you don't need to bother about latter .

 

[NihalRi]

My reaction is to think you need to know what the error of your spectrophotometric measurement is . and my guess is that this is so sufficiently more than the 0.03% is that right? weight error range that you don't need to bother about latter .

Yes, I can get the error from the spectrophotometric measurement which is ±0.0005 from the readings which is around 0.09% for my first reading and 0.48% for my last. So I don't need to bother with calculating the error in the concentration. Got it thanks :)