Sig Figs

[ggxoxo]

There is a qn that EXACTLY says: "A current of 6.00 A was passed for exactly 4 minutes through the solution. During this time, one electrode increases in mass. Determine the mass of solid deposited on this electrode."

Based on this info, how many sig figs would you base your final answer??? I thought it would be 1 sig fig because of the 4 mins... But the book said it was 3 sig figs (which I'm guessing is from the 6.00 A)- so I'm confused, why do we use the 6.00 A and not the 4 minutes???

[thushan]

There is a qn that says: "A current of 6.00 A was passed for exactly 4 minutes through the solution. During this time, one electrode increases in mass. Determine the mass of solid deposited on this electrode."

Based on this info, how many sig figs would you base your final answer??? I thought it would be 1 sig fig because of the 4 mins... But the book said it was 3 sig figs (which I'm guessing is from the 6.00 A)- so I'm confused, why do we use the 6.00 A and not the 4 minutes???

In the VCAA exam, they'll be more rigorous with sig figs. They'll say 4.00 minutes.

[paulsterio]

Ahh, be careful though, certain things are expected to be exact - I remember a question being about "2 drug tablets" being crushed and a friend of mine arguing with me that "2" is actually one significant figure, thus the answer should be to one significant figure...just saying.

[morantz]

That question, which I assume is related to electrolysis, requires you to find the charge (Q) which is current multiplied by time. Current is left as 6.00 amps, whilst time is in minutes and needs to be converted to seconds, thus 4 minutes equate to 240 secs, hence it's 3 significant figures in both cases anyway.

[thushan]

Nah - thing is that we're multiplying 4 (1 sf) x 60 (an exact quantity) = 240 (which TECHNICALLY should be 2.40 x 10^2 seconds). They should say 4.00 in the VCAA exam.

[ggxoxo]

Nah - thing is that we're multiplying 4 (1 sf) x 60 (an exact quantity) = 240 (which TECHNICALLY should be 2.40 x 10^2 seconds). They should say 4.00 in the VCAA exam.

Thanks guys; just one more qn, why should it technically be 2.40 x 10^2 s after multiplying 1 sf with an exact quantity?

[charmanderp]

Like Thushan said I'd interpret it as being 4*60 as minutes isn't a unit of time we can use to calculate Q. Hence we end up with three sig figs. Although this is a bit dodgy.

[Mao]

minutes isn't a unit of time we can use to calculate Q

Yes it is. Unit conversions are exact by definition (well, most unit conversions are), so we can convert minutes to seconds or whatever units we so desire without loss of precision.

I must absolutely stress that converting units does NOT change the precision of a measurement. Ever.
(this is the case where unit conversions are exact by definitions. There are empirical units that cannot be known exactly, one of which is mol, the others you don't really want to care about right now.)

4 exact minutes = 240 exact seconds. The only imprecise quantity here is 6.00A, and so the answer carries the precision of the least precise quantity, in this case 6.00A, so the answer is to 3 s.f.

[thushan]

Yup, I'm with Mao, but only on the premise that the question meant 4 exact minutes, and not the imprecise 4 minutes (so therefore could  be 4.1, 3.9 etc).

[Mao]

But Thushan, we also agree that anyone who claims there is such thing as '4 exact minutes' is a bit delusional. So let's leave it as that.

[charmanderp]

minutes isn't a unit of time we can use to calculate Q

Yes it is. Unit conversions are exact by definition (well, most unit conversions are), so we can convert minutes to seconds or whatever units we so desire without loss of precision.

I must absolutely stress that converting units does NOT change the precision of a measurement. Ever.
(this is the case where unit conversions are exact by definitions. There are empirical units that cannot be known exactly, one of which is mol, the others you don't really want to care about right now.)

4 exact minutes = 240 exact seconds. The only imprecise quantity here is 6.00A, and so the answer carries the precision of the least precise quantity, in this case 6.00A, so the answer is to 3 s.f.

No you're absolutely right, I phrased that really badly.

What I should have said is that within the VCE the formula Q=it is taught to us in units of amps for current and seconds for time, so it saying '4 minutes' implies that we as VCE chemistry students have to convert it to seconds and hence it becomes 240 seconds as an exact quantity, which is 3 significant figures. We can't use 4 on its own as the unit of time for this formula to calculate Q.

On the topic of 'exact', the question does say 'exactly 4 minutes'

[Mao]

hence it becomes 240 seconds as an exact quantity, which is 3 significant figures.

Nope. if 240 is an exact quantities, it has an infinite number of significant figures, not 3.

[charmanderp]

hence it becomes 240 seconds as an exact quantity, which is 3 significant figures.

Nope. if 240 is an exact quantities, it has an infinite number of significant figures, not 3.

Ooh. Noted.

If 240 was inexact would it be 2 or 3? significant figures? Pretty sure it's 2 but I've heard conflicting answers from different sources.

[thushan]

hence it becomes 240 seconds as an exact quantity, which is 3 significant figures.

Nope. if 240 is an exact quantities, it has an infinite number of significant figures, not 3.

minutes isn't a unit of time we can use to calculate Q

Yes it is. Unit conversions are exact by definition (well, most unit conversions are), so we can convert minutes to seconds or whatever units we so desire without loss of precision.

I must absolutely stress that converting units does NOT change the precision of a measurement. Ever.
(this is the case where unit conversions are exact by definitions. There are empirical units that cannot be known exactly, one of which is mol, the others you don't really want to care about right now.)

4 exact minutes = 240 exact seconds. The only imprecise quantity here is 6.00A, and so the answer carries the precision of the least precise quantity, in this case 6.00A, so the answer is to 3 s.f.

No you're absolutely right, I phrased that really badly.

What I should have said is that within the VCE the formula Q=it is taught to us in units of amps for current and seconds for time, so it saying '4 minutes' implies that we as VCE chemistry students have to convert it to seconds and hence it becomes 240 seconds as an exact quantity, which is 3 significant figures. We can't use 4 on its own as the unit of time for this formula to calculate Q.

On the topic of 'exact', the question does say 'exactly 4 minutes'

Oh, perfect. No worries there.