Sig Figs: Converting Celsius to Kelvin & Graphing Moles vs Time

[caljuice]

I want to convert celsius to Kelvin using 273.15. I have a temperature 22.5 C. Then I want to find 1/T.

273.15+22.5 = 295.7 since 22.5 has the least decimal places?
But does 1/T = 1/(273.5+22.5) only have 3 sig figs since 22.5 is smallest sig fig? Or do I use 1/295.7?

Another sig fig problem I have is with graphing.

Moles vs time
.00020 115
.00060 325
.00100 581

Will the slope of the equation of the line be 2 sig figs or 3? My assumption is 2 because of the moles being 2 sig figs. Thanks in advance.

 

[The legend]

http://en.wikipedia.org/wiki/Significance_arithmetic

This might clear your doubts...if not , just ask..

 

[caljuice]

Thanks, but didn't really clear my doubts.
Thinking it over, is the conversion of celsius to kelvin then 5 sig figs because 273.15 is a known constant?

What i don't get about 1/T is which number's sig fig I take because it's addition then division.

 

[Borek]

Addition and subtraction are slightly different.

All digits in 273.15 are exact.

 273.15
+ 22.5
-------
 295.65

After addition last 5 doesn't make sense, as we have no idea what it was added to. The result is 295.7

However, if you need 1/T, best approach is to calculate 1/295.65 and round it down to 4 SD, as you should never round down intermediate results.

Please note - significant figures are not used in a serious work when it comes to express uncertainty of the result, so you shouldn't treat them too seriously. Unless your report will be graded by some nitpicker detached from the lab reality, using 3SD should be OK.

 

[caljuice]

Thanks that helps out. I find chem has a lot more rounding for each step than for physics. Going to be confused for awhile when i go back to physics next semester.