ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: taqi on May 15, 2011, 05:17:50 pm
-
hi,
So far doing chem, significant figures havent been a problem. Simple, answer has the least number of sig figures used in calculations. However so far i thought that rule only applies to given measurements. Like if they give you 20.00ml of 0.400M that equates to 0.00800mol. But i thought that didnt apply to calculations involving molar mass, like 20.00mol of fe=55.9, the answer would be to 4 sig figs, as the 55.9 isnt a measurement. Or is the answer 3 significant figures? I have no clue now.
-
The no. of sig figs required is the least no. of sig figs in the question. So with your example, 20.00ml of 0.400M, i believe you shuold leave your answer as 0.008 mol.
-
The no. of sig figs required is the least no. of sig figs in the question. So with your example, 20.00ml of 0.400M, i believe you shuold leave your answer as 0.008 mol.
I believe that this is incorrect since 0.008 has only 1 significant figure. I think it should 0.00800 mol since this would be the same amount of significant figures as the value with the last amount of significant figures.
As for the molar mass question, I think it should be rounded to 3 significant figures. Even though it isn't a measurement.
-
I believe that this is incorrect since 0.008 has only 1 significant figure. I think it should 0.00800 mol since this would be the same amount of significant figures as the value with the last amount of significant figures.
Yep, that would be correct.
I have a question related to this. Say the question stated "20ml of 0.400M..." would the answer be to 2 sig figs, or are all volumes set to 4 sig figs?
-
Well seeing as 20 is actually only 2 significant figure its probably safe to only put two. However I'm pretty sure they will always give titre and aliquot readings as 4 significant figures so its not a problem
-
I have a question related to this. Say the question stated "20ml of 0.400M..." would the answer be to 2 sig figs, or are all volumes set to 4 sig figs?
[/quote]
Lowest number of sig figs, in this case, 2 or if you are in doubt go to 3 sig figs.
-
I have a question related to this. Say the question stated "20ml of 0.400M..." would the answer be to 2 sig figs, or are all volumes set to 4 sig figs?
Lowest number of sig figs, in this case, 2 or if you are in doubt go to 3 sig figs.
[/quote]
Say we have a question where we use 3 significant figures. If we find ourselves in this situation:
1.00 + 2.00 = 3.00
If our calculator says 3.01 due to rounding, which one do we put down?
I mean sometimes it feels awkward doing:
1.00 + 2.00 = 3.01
-
I have a question related to this. Say the question stated "20ml of 0.400M..." would the answer be to 2 sig figs, or are all volumes set to 4 sig figs?
Lowest number of sig figs, in this case, 2 or if you are in doubt go to 3 sig figs.
Say we have a question where we use 3 significant figures. If we find ourselves in this situation:
1.00 + 2.00 = 3.00
If our calculator says 3.01 due to rounding, which one do we put down?
I mean sometimes it feels awkward doing:
1.00 + 2.00 = 3.01
That's why it's better to only write the rounded answer at the end (i.e. for the final calculation). If it asks for something then hence find something else, write both to correct number of sig figs but then write the unrounded ones when writing your working. They can't really mark that approach wrong, but they might if you write 1.00 + 2.00 = 3.01, as it is mathematically incorrect.
-
say the question included a value thats 2 sig.fig but the calculations you do dont use this value then, would you give an answer in the least no. of sig.fig of the values you used?
-
say the question included a value thats 2 sig.fig but the calculations you do dont use this value then, would you give an answer in the least no. of sig.fig of the values you used?
Yes, because the inaccuracy of the 2 sig fig data has not affected your results.
-
hi,
So far doing chem, significant figures havent been a problem. Simple, answer has the least number of sig figures used in calculations. However so far i thought that rule only applies to given measurements. Like if they give you 20.00ml of 0.400M that equates to 0.00800mol. But i thought that didnt apply to calculations involving molar mass, like 20.00mol of fe=55.9, the answer would be to 4 sig figs, as the 55.9 isnt a measurement. Or is the answer 3 significant figures? I have no clue now.
I know this question was asked a while back, but having a quick skim through the thread I don't think it was ever really answered... Anyway, simple answer to your above question - yes the M(Fe) = 55.9 does count towards significant figures, because it is a measurement! After all, all molar masses are empirically determined, so there is always some degree of experimental error associated with them.
-
I have a question related to this. Say the question stated "20ml of 0.400M..." would the answer be to 2 sig figs, or are all volumes set to 4 sig figs?
Lowest number of sig figs, in this case, 2 or if you are in doubt go to 3 sig figs.
Say we have a question where we use 3 significant figures. If we find ourselves in this situation:
1.00 + 2.00 = 3.00
If our calculator says 3.01 due to rounding, which one do we put down?
I mean sometimes it feels awkward doing:
1.00 + 2.00 = 3.01
That's why it's better to only write the rounded answer at the end (i.e. for the final calculation). If it asks for something then hence find something else, write both to correct number of sig figs but then write the unrounded ones when writing your working. They can't really mark that approach wrong, but they might if you write 1.00 + 2.00 = 3.01, as it is mathematically incorrect.
I see, but what if the last part of the question was 1.00 + 2.00 using the 'rounded ones' and the final answer as on the calculator was 3.01? What should I do then?
-
You should be writing 1.00...+2.00... = 3.01
Even better, state all components of your calculations to 4 dp, then state your answer correct to the required no. of decimal places. Always use the unrounded values in subsequent calculations.
Chem Teacher.
-
I think the rule of thumb is:
- Calculate everything using values of around 4 d.p for accuracy, then leave your final answers using the least number of significant figures given in the question (NOT including numbers you will never use - they are sometimes there as unimportant extra information or to trick you)
- Make sure the answers to EACH question is in the correct significant figures i.e. question 1 or 1a) b) c)
- When continuing on a calculation in the next section (e.g. (b) Hence, use the answer in (a) to...), use the previous answer with the correct significant figures
They're actually rules of thumb, aren't they? Well, that sounds weird. Hope this helps :)
-
I think the rule of thumb is:
- Calculate everything using values of around 4 d.p for accuracy, then leave your final answers using the least number of significant figures given in the question (NOT including numbers you will never use - they are sometimes there as unimportant extra information or to trick you)
- Make sure the answers to EACH question is in the correct significant figures i.e. question 1 or 1a) b) c)
- When continuing on a calculation in the next section (e.g. (b) Hence, use the answer in (a) to...), use the previous answer with the correct significant figures
They're actually rules of thumb, aren't they? Well, that sounds weird. Hope this helps :)
+1. I see; I always thought that you use answers on your calculator to work on the next question instead of using the rounded answers from the previous part. Or doesn't it really matter?
-
+1. I see; I always thought that you use answers on your calculator to work on the next question instead of using the rounded answers from the previous part. Or doesn't it really matter?
Some trial exam solutions I've seen use the rounded answer. I personally just use the value from the calculator for the next part of the question, as it is likely you are going to use that value anyway. I believe this is the better way to go, seeing as it is more accurate.
-
leave as many decimal places in as possible until you reach the final answer!