Worried about sig fig problem (again)

[kenhung123]

So on my exam I should put all values to the correct sig figs? (yes/no?)


If yes, what happens when my working doesn't correspond to my answers e.g. calculator value: 1.51254 mol => sig fig: 1.51
I want to find mass of something so I show this on the paper: m=nM=1.51[this is sig fig value I show on paper, I am actually using 1.51254 on calc]x207.2=313g[rounded to 3 sig figs]

So mathematically, this doesn't make sense because 1.51x207.2=312.87g

Am I doing something wrong? Any suggestions on how I can avoid such confusions?

[Martoman]

Is your problem the fact that if you have say 1.6 in a practical situation you can't round it up to 2 because its impossible to have 'more' than whats there?

If so, don't worry about it (we call this overthinking it happens to me a lot!) If you keep it all on your calc you can round every line to any amount of decimal places so long as your final answer has the correct number of sig figs.

[crayolé]

Okay, so you have two options

-Round to sig figs after every calculation

-Keep your calculator values and round to sig figs at the end


Your problem here is that you are doing both

If you are going to round to sig figs after every calculation, you should use that new rounded figure for everything afterwards. Don't use all those extra numbers you have in your calculator

If you are going to round to sig figs at the end, keep your calculator values and when writing down your steps, include about 4-5 significant figures throughout so it makes sense. Once you have your final answer, you can round it to the correct number of sig figs

[Martoman]

To round after every is absolutely needless and a time waster. It is also another thing to think about. Just keep it in your calc. It has a memory function for a reason.

[kenhung123]

Right, so when the working out on paper does not correspond, it is in fact not acceptable because I am doing both?

Which way is rather more convenient?

[kenhung123]

To round after every is absolutely needless and a time waster. It is also another thing to think about. Just keep it in your calc. It has a memory function for a reason.

Right, so if I use about 4-5 sig figs, normally the working out on page would make sense?

Also, is this acceptable? (This is using the calculator full value method)

E.g. q1a
n(Na+)=CV=0.123*0.31=0.0381 mole (final answer for 1a rounded to 3 sig figs)

q1b
n(Cu2+)=1/2n(Na+)=0.5*0.038068=0.0190mole (final answer 1b rounded to 3 sig figs)

So yea, once again, in 1st step my Na+ mole is different to 2nd step. This would be recognised by examiners or not?

[Martoman]

Yeah. I typically use 2-3 decimal places when working out stuff. It works. And if it is sufficiently large or small, use scientific notation. This communicates with the examiner what you have done, whilst not worrying about the nitty gritty of if you have s.f correct in your 5th line of working.

[Greggler]

i just go through using the calc value. And what i right down on the paper is usually 3 or 4 sig figs. then i finally correct my answer at the end to appropriate sig figs

[stonecold]

Aaargh I hate this shit.  Okay, so I was going through VCAA 2009, and the final question.  Everything is sweet, accept they round after every step whereas I didn't.  So I ended up with 0.20 mg and they got 0.19 mg

I have no idea what to do?  Do I write down all my intermiediate calculations in 5-6 d.p. then round for sig figs at the end.
Or should I just round at every step?  This is what I've been doing.  I write down to the number of sig figs, but then use the calculator value.

I'm so confused.

Also, If a question is more than one mark, then I guess you'd need to show sig figs all the way through yeah, as they could be marking any of the answers...

Edit:  WTF VCAA!  In last years examiners report, they use the calculator value in question 4, and then in question 10 they use rounded values.

What are we supposed to do...

[shinny]

The only answers that need to be in proper sig figs are your final answers to individual questions. Your working out doesn't need to be in any particular form; as long as the examiners know roughly what number you're referring to. To definitely avoid any confusion (and more so to avoid careless mistakes), you should always be writing things like n(Na+)=[Na+]*V(Na+)=3.12*1.5 or whatever. Whether you write 3.12 or 3.15778948 doesn't matter since this way they'll know what number you're referring to. The examiners really aren't going to care. What they do care about is whether you maintain or store the values on your calculator in between questions, which is the main habit you'll need to get in the habit of doing. While I doubt you'll commonly lose marks for even having used the rounded off sig fig values from a previous question since they do take this into account when accepting answers, it's much safer to do it properly.

[chem-nerd]

The examiners often deliberately use rounded figures in the report to make for easier reading. As shinny said they do expect you to maintain the calculator value. And they usually allow some leeway in accepted answers to allow for premature rounding (unless it results in a stupid answer)

[stonecold]

Okay thanks.  Because yeah, usually I often get like 0.01 or 0.001 off answers. 

I usually keep the value in my calculator, so I will keep on doing this.

Is it best to just record values to 5 or 6 d.p. or just round in intermediate workings?

[shinny]

Okay thanks.  Because yeah, usually I often get like 0.01 or 0.001 off answers. 

I usually keep the value in my calculator, so I will keep on doing this.

Is it best to just record values to 5 or 6 d.p. or just round in intermediate workings?

Like I said above, they're not really going to care what you write in your working out whether it be correct sig figs or some number of decimals, as long as they can identify where you got that number from. Only the final answer counts for sig figs.

[stonecold]

Sorry, I would just like something else clarified.  Sometimes you need to use a lower number of sig figs than what is in the question stimulus.  Why is this?  My teacher said it is when you subtract.  i.e. 10.80 g - 3.80 g = 7.00 g

So you go from 4 sig figs to 3, but I still encounter other times when it happens.  How do I know how many sig figs to use?

Also, the only thing that should be taken into account when determining the number of sig figs is the values which you use in your calculations yeah.  If you don't use a value in a specific calculation, then it doesn't determine the number of sig figs.  Is that right?

[chem-nerd]

http://vcenotes.com/forum/index.php/topic,17173.0.html