Hard chemistry calculations

We did an experiment in class and had to do some questions to analyse our data.
The questions are pretty tough to deal with as i've done nothing like them before.
They were:
The balance has a uncertainty of +- 0.01g in each reading. Calculate the percentage uncertainty in your mass of sodium hydrogencarbonate
Explain why hydrated sodium carbonate and sodium hydroxide are not appropriate bases for making up a standard solution.

For the first one I know how to do it but I'm not sure whether i double the value so 0.01 to 0.02 or leave it as 0.01. The mass we used was 2.10g
Any help?

Did you take a single reading of the mass of sodium hydrogencarbonate as 2.10g? If so the percentage uncertainty is just (0.01/2.10) *100