Precision and accuracy of a solution

[cham]

Homework Statement

so let's say that we have a solution S=0.4±0.01/250ml
i want to do a dilution
so i pick 5ml and pour it into 100ml beaker
how should i calculate precision of the solution (mathematically or with some other equations)?

Homework Equations

so basically we have F=5*S/100

The Attempt at a Solution

for me 5*S/100=8*10^-5±2*10^-6
is this true because it doesn't seem right for me (about the pricision)?

 

[mfb]

The relative uncertainty stays the same, that is right.
You might get an additional uncertainty if the 5ml or the 100ml are not exact.

 

[cham]

so

The relative uncertainty stays the same, that is right.
You might get an additional uncertainty if the 5ml or the 100ml are not exact.

so what you say is that no mater how many dilution we do the precision stays the same and it depends on the solution we're using.so it means ±0.01 for all diluted solutions we made,right?

 

[mfb]

The relative uncertainty stays the same.
It is always 1/40. Roughly speaking, your solution might be 1/40 more concentrated than you thought, or 1/40 less. And that uncertainty does not change if your dilution factors afterwards are known exactly.

 

[cham]

so let's assume that final result is true , so it would be 8*10^-5 ±1/40 and so for all resulting solutions
and that by using Δy/y
correct me if I'm wrong
thanks and sorry for my stubbornness

 

[Borek]

I am afraid you can be sure neither 5 mL nor 100 mL are exact and they are additional sources of error.

 

[mfb]

so let's assume that final result is true , so it would be 8*10^-5 ±1/40 and so for all resulting solutions

No, in this case it would be 8*10^-5 +- 2*10^-6 as the uncertainty is 1/40 of the value.

With the caveat that the 5ml and 100ml won't be exact, as mentioned.

 

[Borek]

Also see http://www.titrations.info/pipette-burette for the volumetric glassware tolerances.

 

[cham]

thank you mfb and Borek for your help
much appreciated

 

[Borek]

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