Sig Figs also?

[destain]

Is 9.1 x 10^-6 two sig figs or three?
From the question's information, three sig figs is the lowest that I've used so I assume my answer should be in three sig figs? I was thinking 9.12 x 10^-6? But answers on a VCAA paper says 9.1 x10^-6..

The information I used was pH = 5.04
So i did 10^-5.04 for my answer

[charmanderp]

9.1 x10^-6 is two sig figs. Did you use any other numbers? Acidity constant, maybe?

Which exam/question was it?

[destain]

2009 Q 2c ii)
There is an acidity constant used later on but for ii), you don't need any other information except pH = 5.04 so I thought it'd be 3 sig figs?

[charmanderp]

This is a bit peculiar. I wouldn't worry too much about it though, because with VCAA chemistry exams it's speculated that they often only look at one question for significant figures. In the case of the 2009 exam it's probably Question 7b. given that it ends with 'Express your answer to an appropriate number of significant figures'. So always try to be accurate with sig figs but this is perhaps how the marking system works, so be especially precise if you ever see this wording tacked onto a question. For other questions just try and give your answer to a reasonable number of sig figs.

[destain]

ah ok but usually you'd give it to the lowest number of sig figs in the information that you've used for the question isn't it?

[charmanderp]

Lowest/least accurate, yes.

[d3stiny]

2009 Q 2c ii)
There is an acidity constant used later on but for ii), you don't need any other information except pH = 5.04 so I thought it'd be 3 sig figs?

For pH, 5.04 is 2 sig figs
Refer to 2011 assesor's report Q 6bii, the number before the decimal point is not considered. However 1 sig fig (5.0) would generally be accepted. 10^(-5.04) would not though.

[destain]

2009 Q 2c ii)
There is an acidity constant used later on but for ii), you don't need any other information except pH = 5.04 so I thought it'd be 3 sig figs?

For pH, 5.04 is 2 sig figs
Refer to 2011 assesor's report Q 6bii, the number before the decimal point is not considered. However 1 sig fig (5.0) would generally be accepted. 10^(-5.04) would not though.

Sorry, I don't really understand that except for the part where 5.04 is seen as 2 sig figs but i don't understand why

[charmanderp]

Surprisingly this is actually true. See the bottom of this page: http://web.campbell.edu/faculty/fetterman/Significant%20Figures.htm

That's crazy, I've never been told anything like that before. Like I said previously though this is sufficiently ambigious that VCAA PROBABLY wouldn't have used this question to account for the significant figures mark.

[destain]

Is it saying in pH calculations (logs), you ignore the first digit in this case, 5? and so it's only two sig figs?

[charmanderp]

Apparently - I'm hunting around for a more defined source. Might have to wait for Mao/Thushan to come on here, otherwise.

[d3stiny]

Is it saying in pH calculations (logs), you ignore the first digit in this case, 5? and so it's only two sig figs?

Yeah, basically the first number only 'locates' the decimal point so the decimals are only considered. Don't worry about it too much though, just make sure you evaluate expressions like 10^(-5.04) to a decimal.

[destain]

TIS BEYOND ME!