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uncertainity errors

to find the error in the balance, why did it multiply (0.005/5)*100 (5 is the value of mass given in question) by 2???
3a.PNG20.0KB

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Because to find the percentage uncertainty of an instrument, we do the (error/reading)*100
Yup then it multiplied the whole thing by 2.. Why was the balance reading even read twice to have a sensitivity error multiplied by two - all it mentioned in the Q was the mass as 5g
Reply 3
Avatar for Pigster
Pigster
20
Was the mass found by difference, i.e. was there a container with the solid in at the start (mass found), then the solid tipped into something else, followed by finding the mass of the now empty container. The mass of the solid is the difference of the two masses.

That'd explain it - you're using the balance twice, hence 2x uncertainty.
Original post by Pigster
Was the mass found by difference, i.e. was there a container with the solid in at the start (mass found), then the solid tipped into something else, followed by finding the mass of the now empty container. The mass of the solid is the difference of the two masses.

That'd explain it - you're using the balance twice, hence 2x uncertainty.

but can we make this assumption that the difference is taken even tho that wasnt mentioned.. heres the question
all it says that 5g were taken nthn about how it was prepared...
uncert..PNG61.2KB
Reply 5
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Pigster
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Do you have the M/S that I can look at?
Original post by pondsteps
but can we make this assumption that the difference is taken even tho that wasnt mentioned.. heres the question
all it says that 5g were taken nthn about how it was prepared...


I know for a balance, we always multiply the percentage uncertainty by two because the mass is always takes as initial and final mass unless the question have stated that we only took the final mass but can you send the year of this paper, I will look it up
Original post by Pigster
Do you have the M/S that I can look at?

yeah here u go
% error.PNG27.4KB
Original post by PlayerBB
I know for a balance, we always multiply the percentage uncertainty by two because the mass is always takes as initial and final mass unless the question have stated that we only took the final mass but can you send the year of this paper, I will look it up


are u sure u are talking about mass, not volume?
and it's from 2015 june unit 3b - i guess this is the paper you set for the external lol
Original post by pondsteps
are u sure u are talking about mass, not volume?
and it's from 2015 june unit 3b - i guess this is the paper you set for the external lol


Ahh right, sorry I got confused, that's normally for the volume. Have they specified in the question that they have measured it twice or something ?

Yeah actually it is and surprisingly I got 60 ums in it :biggrin:

Posted from TSR Mobile
Reply 10
Avatar for Ayman!
Ayman!
15
Original post by PlayerBB

Yeah actually it is and surprisingly I got 60 ums in it :biggrin:

Posted from TSR Mobile


:five:

Need this in 10 days as well. :lol:
Original post by Ayman!
:five:

Need this in 10 days as well. :lol:


Yeah!! We have to smash it :five:
There must be some guidance somewhere that states that when you measure out a mass, you use the balance twice and do a subtraction to find out how much actually was transferred.

Even if you zero'd the balance with a beaker on it and added 5 g of powder to the beaker, there is are still two measurements, i.e. one at 0.00 g +/-0.005 and 5 g +/-0.005
Original post by Pigster
There must be some guidance somewhere that states that when you measure out a mass, you use the balance twice and do a subtraction to find out how much actually was transferred.

Even if you zero'd the balance with a beaker on it and added 5 g of powder to the beaker, there is are still two measurements, i.e. one at 0.00 g +/-0.005 and 5 g +/-0.005

if it has not mentioned any of this, only the mass is given can I assume that this has been done and x2 ??
all the Q says is 5 grams of powdered amonium chloride is added to water ...
(edited 7 years ago)
Original post by Ayman!
:five:

Need this in 10 days as well. :lol:


Original post by PlayerBB
Yeah!! We have to smash it :five:


good luck :smile:
Original post by pondsteps
if it has not mentioned any of this, only the mass is given can I assume that this has been done and x2 ??
all the Q says is 5 grams of powdered amonium chloride is added to water ...


Like I said, there must be some exam board specific guidance on equipment uncertainties when using a balance.
Original post by Pigster
Like I said, there must be some exam board specific guidance on equipment uncertainties when using a balance.

yup I just searched it up. I think I should always multiply the percentage uncertainty of the balance by 2.
thanks for helping out!
Original post by pondsteps
yup I just searched it up. I think I should always multiply the percentage uncertainty of the balance by 2.
thanks for helping out!


No, be careful! I just solved a questions where I multiplied the error by 2 for a balance and it was wrong, I shouldn't have so I searched June 2015 paper 3B, the paper you posted the question from, it says, find the percentage error of each reading which directly means the the balance have been used twice!! So we multiply the error by 2
Original post by playerbb
no, be careful! I just solved a questions where i multiplied the error by 2 for a balance and it was wrong, i shouldn't have so i searched june 2015 paper 3b, the paper you posted the question from, it says, find the percentage error of each reading which directly means the the balance have been used twice!! So we multiply the error by 2


thankkkkkkkkkks SOOO MUCH
didn't pay attention to thattt
Original post by pondsteps
thankkkkkkkkkks SOOO MUCH
didn't pay attention to thattt


No problem :h: glad I helped!!

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