Confused about sig figs

[zsteve]

I went to Thushan's revision lecture at RMIT during the holidays (and also the KLD/UMEP ones), and I was alerted to additional rules sig figs in relation to log and exp, namely:
- Result of log - sig figs become decimal places
- Result of 10^x - decimal places become sig figs.

Trial exams and my teacher aren't too familiar with these, so pH calculations, etc. all go weird. I would like to know - does VCAA care about these when calculating pH? Would you lose a mark?

Also, how do you cope with addition/subtraction of decimals where not all decimal places are significant? This might be a simple question, but I've forgotten.
For example, in NEAP 2014:
A starch molecule is composed of 2000 glucose monomers (M = 180) find the molar mass of this molecule.
M = 2000(180)-1999(18.0) - my working.
Now what? How do I cope with sig figs?
2000 is an integer so it has no effect on sig figs, as is 1999. So the two additive terms come to 3 sf.
M = 3.60*10^5 - 3.60*10^4.
I state my answer to the least (significant) decimal place:
M = 3.24*10^5
Is this correct? Are there any faster ways of doing this?

And if they ever give you data to one sig fig, what do you do?

[wunderkind52]

I went to Thushan's revision lecture at RMIT during the holidays (and also the KLD/UMEP ones), and I was alerted to additional rules sig figs in relation to log and exp, namely:
- Result of log - sig figs become decimal places
- Result of 10^x - decimal places become sig figs.

Trial exams and my teacher aren't too familiar with these, so pH calculations, etc. all go weird. I would like to know - does VCAA care about these when calculating pH? Would you lose a mark? From my experience there have been some questions in the past where they've specifically awarded marks for significant figures, which to me says you can afford to muck up your significant figures except for that 1 question. I don't think you can lose lots of marks for it, it's probably like rounding in methods/spesh. I have a feeling they'll be a bit more lenient with pH calculations because lots of people don't actually seem to be aware of those rules...

Also, how do you cope with addition/subtraction of decimals where not all decimal places are significant? This might be a simple question, but I've forgotten. Your answer should have the same number of decimal places as the initial number with the lowest number of decimal places. I.e. 2.569-0.3=2.3 (0.3 has 1dp so your answer has 1dp as well, which makes 2sf in this instance)
For example, in NEAP 2014:
A starch molecule is composed of 2000 glucose monomers (M = 180) find the molar mass of this molecule.
M = 2000(180)-1999(18.0) - my working. It should be 180.0 i think (unless they told you it's 180gmol^-1.
Now what? How do I cope with sig figs?
2000 is an integer so it has no effect on sig figs, as is 1999. So the two additive terms come to 3 sf. That's right, only experimentally derived values affect sf.
M = 3.60*10^5 - 3.60*10^4.
I state my answer to the least (significant) decimal place:
M = 3.24*10^5
Is this correct? Are there any faster ways of doing this? yeah i think that's right. It's tedious, but I don't know any faster method :/

And if they ever give you data to one sig fig, what do you do?

[Fullmethyl Alchemist]

I'm pretty sure VCAA don't care about sig figs in the context of calculating PH (but yes, number of sig figs=number of decimal places). Not sure about exponentials though...

Number of sig figs is not usually number of decimal places. 500 has 3 sig figs. 500.0 has 4. 5.0*10^2 has 2.

Generally as long as your answer is good the examiners are not going to be looking in your working. I'd find the lowest sig fig in the question (eg 1.00 mol of blah was mixed with 0.25 mol of meh, the sig figs you'd want is 2) scribble it down somewhere and give all the answers to that question to 2 sig figs.

(But yeah they don't normally care for pH)

[lacune]

I'm pretty sure VCAA don't care about sig figs in the context of calculating PH (but yes, number of sig figs=number of decimal places). Not sure about exponentials though...

Sig figs is an expression of how accurate sth is. Number of sig figs with exps and logs is the number of digits in the mantissa (the numbers after the decimal point), not the character or in total, as the character (digit before decimal point) only gives an indication of magnitude.
Put more simply for pH, if [H30+] = 0.014M (2sf), then pH = 1.89 (2sf) and NOT 1.9 (only 1sf)