Sig. Figures/Decimal Places in calculations. (1 Viewer)

[asdfqwerty]

Been meaning to ask this for a while, but can somebody please explain to me the importance of significant figures and decimal places in calculations and the conditions in which they are applied? I just looked back at my Prelim results and I've lost quite a few marks because of it; My teacher never taught us in class.

Thanks .

 

[Dylanamali]

give your answer to the lowest amount of significant figures displayed in the question.

 

[asdfqwerty]

give your answer to the lowest amount of significant figures displayed in the question.

So if the question is ' 15.675g of NaOH is diluted blah blah blah and 3.8 g of HCl is needed to reach the equivalence point, calculate etc etc...if I were to find mol/L, do I have to write say for example 2.4 mol/L opposed to 2.396 mol/L since 3.8 is the lowest amount of sig. figures given to me?

 

[Dylanamali]

Yes that is correct
so you look at each of the values given in the question. The value that is given to the least amount of sig figs is the same amount that you will present in your answer.
So in your example 3.8 is the value with the least amount of sig figs.. 2 sig figs,
therefore you would give your answer to two sig figs.

 

[asdfqwerty]

Okay thanks! what about decimal places? lets say i calculated moles of something halfway and got 0.03876 mol, can i round it up to 0.039 mol and then use 0.039 for my next calculations? and how many decimal places am
i meant to put in?

 

[deswa1]

Never round anything until the end otherwise you lose detail. The easiest way to do it is to keep it stored in your calc. When I'm doing exams, I tend to write something like 0.039 (full answer kept in calc and used) to show that marker that your working is correct to get half the moles but that you are still using the exact value.

 

[Dylanamali]

oh right.. what I do/did, was give it to a reasonable amount..
so during my calculations I'd probably write it to 4 decimal places, but in the actual calculations I'd use the exact answer. It depends how long the answer is on the calculator, its best to write out as many numbers as you can, but obviously if there are heaps you just give it to a reasonable amount. But make sure you use the exact values that you have in your calcultor by pressing "Ans".

 

[someth1ng]

Yeah, you write it on the paper at 2dp or so but ALWAYS use the exact value in the calculator at all times because the difference from rounding can be multiplied up and be quite significant.