[wilmas1]

I was asked to measure a rectangle and came up with the measurements of 14.8L and .7W. When I calculate the area I get the answer of 10.36cm2. BUT, I then need to produce the rounded calculated area. Based on the significant figures in my calculation .7, my answer should contain 1 (one) significant figure...right? But how does that work with the 10.36cm2? 10.cm2? or 10.4cm2? Or just .4cm2?  Please *delete me* Thanks!!!

[bromidewind]

Significant figures vary from course to course.. I took general chemistry from a physical chemistry teacher, so we used the significant figures for physics. Regardless, the basic structure is the same.

If you are actually physically measuring something, then you should estimate between the smallest lines. For instance, if you have a ruler that goes up to millimeters, then you should be able to estimate where your line falls in between those lines. Say this gives you a distance of 136.5 mm. The 0.5 is the digit that you are unsure of, but you are sure of the first three. So you have three significant figures.

Based on the measurements you have provided, you will actually have two significant figures (0.7 cm has two). So your answer would be 10 cm².

I've been dealing mostly with sig figs in physics lately and not so much in chemistry, so I might be wrong. But I hope this at least helps.

[billnotgatez]

So your answer would be 10 cm2.

Are you sure???

[Borek]

Significant figures vary from course to course.

No.

you will actually have two significant figures (0.7 cm has two)

No.

[Borek]

my answer should contain 1 (one) significant figure...

Hint: scientific notation.

[bromidewind]

Significant figures vary from course to course.

No.

you will actually have two significant figures (0.7 cm has two)

No.

It must just be my school then that has differences in sig figs then. My physics class has different sig figs than my organic chemistry class. It's awful when writing formal lab reports, because I keep getting them switched up. Sorry for any confusion.

[stewie griffin]

I think bromidewind is suffering the same fate I suffered. Although there is a correct way to do sig figs, I don't know which of the several ways my professors taught is the "correct" one. It seemed like in general chemistry my professor liked us to do sig figs one way, then my analytical prof liked it done another way, then my physical chemistry prof had her way...
They probably are the same (maybe one prof's method is just a short cut). I dunno. I also don't really care. In synthetic organic we just seem to do 3 sig figs. Sloppy and wrong I know... but that's how most synthetic chemists do it (not all, but most).

[Borek]

Hm, can you give examples? I was always under impression that basic rules are clear.

Significant digits are elaborate method of getting wrong answer

http://www.av8n.com/physics/uncertainty.htm

[stewie griffin]

Well the rules probably are clear but I just never cared much for sig figs. 
In general chemistry we learned the rules about how many sig figs carry over when you add/subtract and when you multiply/divide. We were asked to to every calculation stepwise simply applying each of those rules. In analytical chemistry there was some short cut we used where basically you can do the entire calculation in one step (meaning, if you're solving for x you get x by itself and then do the calculation rather than do a calculation everytime you move something from left or right of the equal sign). In physics we were specifically told that for all our exams we should use exactly 3 sig figs, no more no less. In physics lab we did error bars and propogation of uncertainty. In physical chemistry, I don't even remember being told what to do with sig figs or if we cared.
All I know is that in lovely organic class, we never mentioned sig figs. It was nice.

[bromidewind]

My gen chem prof (who has his doctorate in physical chemistry) had us actually count zeroes to the right of the decimal (and before the digits) as significant. So 0.0007 would have four sig figs by his rules. My physics prof has us not count these, so 0.0007 is only one sig fig. We also do some weird rounding in there where if the first number is even and the second >5, you round up while if it's odd, you round down. So 0.85 is 0.9 while 0.75 is 0.7. The scientific notation in there is weird too. My ochem prof has us just do sig figs when we're measuring things via thermometer, ruler, etc. Anything that you use your eye to measure, we toss sig figs in there. So you can see where my confusion regarding sig figs comes from.. bad teaching and no justification of the "right" method.

[Borek]

0.0007 is 1 significant digt, counting 4 makes it an obvious case of PhD being an idiot.

Rouding trick is used to not skew the sums. If you always round 5 up, sum of rounded numbers will be higher than it should be. Roudning half numbers up and half number down you avoid this problem. Better solution: don't round intermediate results, or use so called guard digits (that is, throw in to or three additional digits at the end of the number).