Significant Number Problem!

[kevinkhoo1986]

Hey guys... Please help me. I have a freaking teacher who like to deduce mark just because i have an extra/less Significant figure on my answer. I will like to know during STPM, do they really care about significant figure? Typically i will put 3 sf during exam.

Here is the example.(chemistry)
3600/96500 = 0.037
But the teacher said it was 0.0373 !!
And out of 3 marks i just managed to get 1 mark because of this SF problem.

BTW 3600 is two SF rite?

[chiunlin]

The question is a bit vague, but I guess you could deduce the correct SF if all the required information is given. 3600 may be 2,3 or 4 SFs, depending on the instrument you used to measure it. Similarly, 96500 may be 3,4 or 5 SFs. In performing the division, you must round your answer to the less significant of the two, so I'm guessing here that 3600 must be at least 3 SFs.

Or maybe it is a rule for Chemistry in STPM to write till 3 SFs.

[Penny]

In Chemistry STPM, they demand precise significant figure/decimal places.

Upon multiplication/division, the final answer will need to be of the lowest significant figure.

For example:
By using the syringe method, 0.120g of a liquid Q can replace 47.0cm3 of air at 100 C (degree celcius) and 1 atmospheric pressure.
Calculate the relative molecular mass of Q.
[1 atmospheric pressure = 101325 Pa, R = 8.30 J/(K mol)]

pV = nRT
(101325)(4X10^-6) = (0.120/Mr)(8.31)(100+273)
Mr = 78.1 (3 sf)

Upon addition/substraction, the final answer will need to be of the lowest decimal place.

Also, the substitution must be right.
In the example, the mass given in the question is 0.120g.
If you substitute 0.12 into the equation, it's wrong.
It must be 0.120.

This is what my teacher taught :lol:

[kevinkhoo1986]

Here is another problem....

2 moles of electrons are involved in the yield of 2 moles of NaOH
2(96500c) will yield 40g NaOH
Hence, 84600c will yield 2(40)/2(96500) x 84600 = 35.1 gram NaOH

Hmm... why not 35 gram only since 40 is 2 SF ? I got that sample question from a book.

[Penny]

Upon multiplication/division, the final answer will need to be of the lowest significant figure.

Lowest significant figure as in the significant figure of the readings which aren't whole number.
Whole number isn't taken into consideration.

For your question, if all the figures are in whole number, I've consulted my teacher, and she said, in this case, you'll need to give 3 significant figure in the answer.

Hope it's clear.

Hmm... it is so confusing..... penny can i have ur msn? Hope that you are willing to help me with this problem...

[Penny]

Ya I do MSN.

it's really very confusing
my teacher said it is based on the info in the question , rite?

Kevin here... lazy to log in :D

I have another question.....
if x = 3.2^-5, y =3.25^-4, z=2.5^-3
so the answer should be in X.X instead of X.XX rite?

Does anyone bought the STPM topical practice exercises for physical chemistry by Federal publication? (written by krisnasamy and samynathan)

The more i do the questions the more i get confused with the significant figure given by the answer. Some of the answer given until xxxx.xxx !!! it's seven significant figure....

[Penny]

Should be of 2 significant figure if it's of multiplication or division.

[vseehua]

i think it would be better if you can write the results in terms of powers of ten like 1.15 x 10 (power 3)

then adjust the s.f. of the numbers as you like..

i found that to be quite effective in my A-levels..hooe that helps :)

what about in physics? My physics teacher said we just need to put 3 sf in every answer that we found... It is not as complicated as chemistry... but i am quite sceptical about it... Izzit true that we just need to put 3 sf in every answer that we found in PHYSICS??

[vseehua]

for most of the questions, 3 will be enough...

with one exception

when calculating the masses of sub-atomis structure (ie protons, neutrons, electrons) use the numbers as they are, and never round it up until u got the the answer...that's what my lecturer had told me...if they giv you 6 sig no, then the answers in u should be 6 sig num too...

[Penny]

I'm not sure for Physics' too.
I screw my Physics anyway :P
My teacher has been pretty lineant and he doesn't stress much on significant problem.

Agree with vseehua, for questions dealing with sub-atomic structure, we dont normally round up till the end of the calculation.

[sping]

I'm not sure about STPM, but for A-levels 3 s.f. should be fine if you are not sure of how many s.f. to use. The rationale is that they have already tested us on s.f in the practicals (aka SPA in Singapore) therefore they are not really particular about it in the exam. Maybe it's also like that in STPM?

[chlamy]

1.22 kJ=1.22x10^3 J ( not 1220 J )
calculation of pH
must be 3 s.f.
pH 2.00
how abt s.f. 4 sketching graph?