How many significant figures? (1 Viewer)

[wrxsti]

How many decimal points and significant figures should we round off our answers to?
i use the whole calculation (store on calculator) and when i check answers mine are lik a bit off cause they round off .... :S

 

[beentherdunthat]

How many decimal points and significant figures should we round off our answers to?
i use the whole calculation (store on calculator) and when i check answers mine are lik a bit off cause they round off .... :S

I always use the whole number 0.0387393 etc. until I get the final answer. And then round off. Sometimes, it makes a difference and they won't give you a mark for continued error.

 

[Captain Gh3y]

As many as the question gives you the original data to, since any more and it's completely meaningless

 

[wrxsti]

um so ill use whole calculation till end? where i round off to figures they give me?

 

[beentherdunthat]

um so ill use whole calculation till end? where i round off to figures they give me?

Yeah, that's what I suggest. They don't really tell you to round off in chem though, not as much as in Maths anyway

 

[undalay]

Your final answer should be to the amount of significant figures equal to the lowest in the question.
Eg what is the weight of MgO after burning 250g of Mg, your answer would be to 2 significant figures.

Exception to this is pH, where you add one. What is the pH of 1 mol of HCl. Your answer would be to 2 significant figures.

 

[xiao1985]

Your final answer should be to the amount of significant figures equal to the lowest in the question.
Eg what is the weight of MgO after burning 250g of Mg, your answer would be to 2 significant figures.

Exception to this is pH, where you add one. What is the pH of 1 mol of HCl. Your answer would be to 2 significant figures.

Almost correct, /clap nontheless

Your final answer should be to the amount of significant figures equal to the lowest in the question.

- this is correct

Eg what is the weight of MgO after burning 250g of Mg, your answer would be to 2 significant figures.

- This is actually ambiguous... the question could be in 2 sig fig or 3 sig fig....

pH is actually different.... pH of 0.1 actually counts as 1 sig fig, believe it or not...


Now, I would carry as many dp as possible, until the final answer, then use the smallest sig fig as stated in question.

 

[Chinmoku03]

I tend to use the entire number given by the calc until I reach the final answer, where I usually round off to 3dp.

Is significant figures the notation BOS prefers? Will I be marked down for answers that are off cos I used decimals instead of significant figures? ;

 

[brenton1987]

Is significant figures the notation BOS prefers?

No. They are different things.

Will I be marked down for answers that are off cos I used decimals instead of significant figures?

Yes because you will give the wrong answer.

eg. 256.265 has 6 significant figures but only 3 decimal places.
If the answer asked for 3 sig. figs. your answer would be 256.
If the answer asked for 3 d.p. your answer would be 256.265.

 

[pattii]

round off at the end
and use as many sig figs as in the Q

 

[me121]

but how do you know the number of sig. fig. in the question. You only know this if all values are given in scientific notation. for example 250g, could be either 2sig. fig. or 3 sig. fig. We don't know.

 

[yoakim]

It's usually 3, so stick with it if it's undecidable - as stated by Roland Smith himself (from somewhere I can't remember!)

 

[undalay]

but how do you know the number of sig. fig. in the question. You only know this if all values are given in scientific notation. for example 250g, could be either 2sig. fig. or 3 sig. fig. We don't know.

Okay guys. 250g this is 2 sig figs. if they want you to use three there will be a dot after it. for example

250 g = 2 sig figs
250. g = 3 sig figs.

pH of 0.1 actually counts as 1 sig fig, believe it or not...

I don't understand :S why wouldn't it count as 1 sig fig?

This is how i learnt it anyways.

 

[Shakugan]

I have also been worrying about this. I do not wish to lose a significant amount of easy marks because of incorrect decisions on how many significant figures.

The questions that always confuse me in regards to significant figures are the questions that include information listed with one significant figure.

For example, a question in the 2002 paper states "A sample of hard water contains 6x10^-4 mol/L of magnesium carbonate. Calculate the mass of magnesium carbonate in 150ml of this sample"

The answer is about 7.59mg, but shouldn't this question be report to 1 significant figure? As 6x10^-4 is only one significant figure. I'm always confused by this because 1 significant figure in this scenario seems very inaccurate.

 

[Forbidden.]

If 'n' amount of significant figures was given in the question then you must use 'n' amount of decimal places in your final answer.
Throughout the calculations you may write down 'n' nubmer of decimal places but keep the full calcualtor display and use it throughout subsequent calculations.

 

[helper]

You are not going to be penalised multiple times for significant figures. They may decide to deduct in one question but it will not worry about it for most questions.