chemistry & Physic [significant number]

Dicardo

chemistry & Physic都要跟significant number嗎？

有些人說要，有些人說不要....

我都亂了。。。

summer_lover

绝对是要跟的.

meiling

是的，一定要跟的。。
不过我发现到的是physic在计算中可以放多一个sf，可是chem是100%从一开始就要跟sf

bell_25

那请问是3个 sf 吗？

chee_yong88

FOR CHEMISTRY. multipication and devision follow the least sig.
plus and minus followw the least decimal pt.
FOR PHYSICS.if whole no. at least 2 sig
whole no.can put 1 decimal pt.
other decimal no. put 2 decimal pt is 100%save.sometime 1 and 3 dec also accepted too.
attention!!!!!!!!!!!!!!!!
no. mean number.

bell_25

原帖由 chee_yong88 于 19-11-2009 08:56 PM 发表
FOR CHEMISTRY. multipication and devision follow the least sig.
plus and minus followw the least decimal pt.
FOR PHYSICS.if whole no. at least 2 sig
whole no.can put 1 decimal pt.
other decimal no ...

那么请问所谓的least sf 和 decimal point 是指多少个sf和dp呢？
谢谢

meiling

当然是看题目中给的value咯

phoebie

我老师也是讲跟问题的。。。问题给几个就几个。。。

darksider

Yeah chee yung is right.

For multiplication and division, our *final answer must follow the least significant figure. So what least significant figure is this? We can know the least significant figure through the given info.

Eg, for eg, 8.0 g of X is dissolved in a solution, if the volume of the solution is 100cm^3, find out the concentration of X. (Mr of X = 50)

So the least significant here is 1 (from the value : 100cm^3 ; 8.0 g has two significant figure)

So your final answer must be in 1 significant figure.


For addition and subtraction, follow the least decimal places.
If A has 0.005g of NaCl and B has 0.0005 g of NaCl, how much g of NaCl does A have more than B?

A - B = 0.005 (3d.p) - 0.0005(4d.p)  = 0.0045 = 0.005 (3.dp)

3 d.p is the lowest decimal place, so 0.0045 is rounded off to 0.005 which has 3 d.p.

-----------------------------------------------------------------------------------------
Rule 1:  Any number that isn’t zero is significant.  Any zero that’s between two numbers that aren’t zeros is significant.

All this means is that if you have actual numbers written down (or zeros between these numbers), they have actual meaning and give you meaningful information.
Example: 198, 101, and 987 all have three significant figures.


Rule 2:  Any zero that’s before all of the nonzero digits is insignificant, NO MATTER WHAT.

Basically, this applies to numbers that are very small decimals.  For example, if you have the number 0.000054, there are only two significant figures (the 5 and the 4), because the zeros in front are insignificant.
But Mr. Guch, don’t those zeros tell me something?  Yes and no.  The reason that you don’t count these numbers as significant is mainly because of rule 4, which we’ll talk about after…


Rule 3:  Any zero that’s after all of the nonzero digits is significant only if you see a decimal point.  If you don’t actually see a little dot somewhere in the number, these digits are not significant.

Let’s consider the numbers “10,000 lbs” and “10,000. lbs”.  The first number is significant only to the nearest ten thousand pounds (only the first “1” is significant) and the second is significant to the nearest pound (all five digits are significant).  What the addition of the decimal does is tell us how good our measuring equipment is.  The first number, for example, was probably taken by a truck scale (which wouldn’t have much use for measuring things to the nearest pound) and the second was probably taken by a bathroom scale (which requires much greater precision).


Rule 4:  When you write numbers in scientific notation, only the part before the “x” is counted in the significant figures.  (Example, 2.39 x 104 has three significant figures because we only worry about the “2.39” part).

Let’s go back to rule #2, in which we said that “0.000054” had two significant figures.  The reason for this is that if we convert it into scientific notation, we end up with the number “5.4 x 10-5”.  If we said that all the zeros in front were significant in rule 2, then we’d have the weird case where the same number had either two significant figures or seven significant figures, depending on how we write it.  Since that kind of confusion isn’t too cool, we ignore them in significant figures to make our lives easier.

Dicardo

可是。。 我看了很多學校的trial..他們的skema答案都沒跟的。。

Dicardo

可是。。 我看了很多學校的trial..他們的skema答案都沒跟的。。

darksider

How to Solve Word Problems in Chemistry

2.2 Significant Digits
No measurement can be made perfectly. Every measuring instrumenthas
a limitas to how precisely itcan be read. For example,
we would never try to measure the length of our shadow with an
automobile odometer (mileage indicator). Scientists attempt to read
every instrument to one-tenth the smallest scale division. Thus a
meter stick with 1000 division marks (that is, 100 centimeters, with
each centimeter divided into 10 parts) can be estimated to 0.1mm.
When a scientist reports the results of the measurement, the scientist
uses as many digits as necessary to indicate how precisely the
measurementwas made. The scientistmightreport0.0531 m to report
a length. We have to recognize which of these digits record
the precision of the measurement, which are present only to specify
the magnitude of the answer, and which do both. If the digit helps
reportt he precision, itis called a significant digit or a significant
figure. The word significant in this sense does not mean important; it
means having to do with precision! Every digitser ves to reporteit her
the magnitude or the precision of the measurement, or both. If the
digit reports the magnitude only, it is nonsignificant.
Significant Digits in Reported Values
First we must learn to recognize which digits in a properly reported
number are significant. They include all nonzero digits. Zeros are
determined to be significant or not according to the following rules:
1. All zeros to the right of all other digits and to the right of the
decimal point are significant. For example, in 1.200 cm, the zeros
are significant.

2. All zeros between significant digits are significant. For example,
in 1.003 cm, the zeros are significant.
3. All zeros to the left of all other digits are not significant. For
example, in 0.022 cm, the zeros are not significant.
4. All zeros to the right of all other digits in an integer cannotbe
determined merely by inspection to be significant or not. For example,
in 1200 cm, the zeros are undetermined without further
information. (Some texts assume that these zeros are not significant,
and they add a decimal point at the end to signify that all
the trailing zeros are significant.)

chee_yong88

physics and chemistry are different.
dint follow the it, marks will be cutted.if the number that you write are different from question ,no mark will be given.
eg in question state that p=10kpa v=2m n=1 t=how many k
when during calulation you write
pv=nRT9(1%)
10000*2=1*8.31*T(wrong 0%)instead of p=10*10009(correct)because the question give us P=10 so you must write 10 to the power of

*mean multiply

chee_yong88

sorry i type wrongly.the correct value of p you must write is 10 to the power of 3 as it change from kpa to pa.,not straight alway 10000pa