Help With Sig Figs

[RKTR]

1.let's say a question needs many steps, do we round off every step or only the final step?

2. if a question has many parts, for example a,b and c, we need ans from a for part b, do we use the round offed ans from a or value in the calculator?

[lArcdeTriomphe]

from what I know:
1. you round off ONLY in the last step. in the intermediate steps, always express the answers to more than the necessary amount of sig figs (or else you will have rounding errors).
2. If the question says "using your answer from part a)," then use the rounded value. If it doesn't specify, I would use the value from the calc (i.e. the more precise answer) - generally examiners/teachers will accept both unless there is a specific requirement

[RKTR]

do we need to consider the values from periodic table? if all other values are 4 sig figs but periodic table shows 3 sig figs, will this affect the sig fig of final answer?

[drake]

YES, molar mass affects significant figures! i think on the periodic table all molar masses are stated to one decimal place (e.g. 20.1 which is three significant figures or 120.1 which is four significant figures) so make sure you take this into account when doing your calculations.

also, as for your questions before, i suggest that whenever you are doing your steps, you always write your working out to the correct number of significant figures. HOWEVER, make sure you keep the EXACT value in your calculator (find a calculator that enables you to do so).

[RKTR]

thanks guys . i still have more questions.

 if ans for part a is 0.4247 then round off to 0.425. part b is half of part a so we use 0.4247/2 ? i'm confused cause sometimes when i'm doing textbook questions ,if i do it like 0.4247/2 i get 0.212(3 sig figs) instead of textbook answer 0.213. 

another question- mass equal no.of mol x molar mass ya?
                               let's say no.of mol is 0.2222 then molar mass is 27.0+35.5+35.5+35.5 =107.5g/mol
                               for sig figs of mass , is it 3 or 4? 3 because the molar mass of each element is 3 sig figs or 4 because total molar mass
                                 is 4 sig figs?       

[Ancora_Imparo]

Hopefully this helps clear some things up: THE FOOLPROOF GUIDE TO SIGNIFICANT FIGURES

another question- mass equal no.of mol x molar mass ya?
                               let's say no.of mol is 0.2222 then molar mass is 27.0+35.5+35.5+35.5 =107.5g/mol
                               for sig figs of mass , is it 3 or 4? 3 because the molar mass of each element is 3 sig figs or 4 because total molar mass
                                 is 4 sig figs?       

It would be 4 significant figures as the total molar mass is the quantity in your equation, not each individual atomic mass. So you would calculate your total molar mass first and then use that value in your equation.

[RKTR]

thanks a lot guys~~