Don't know if anyone can help, but surely one of you must have done this fairly recently, it's on the OCR/Salters syllabus

My daughter for her coursework has been titrating sodium hydroxide into various different water samples until the pH reaches 8.3 & similarly with sulphuric acid until pH 8.3 is reached (coming down from an alkaline sample) & then pH 4.5 is reached.

She's been given the question why are the values of 8.3 & 4.5 used?

I've been thro' all her course info with her, we've searched the internet & asked her brother who did chemistry "A" level only last year & got an A grade (different syllabus) & can't come up with any answers, to us they appear arbitrary values???

Any ideas on answers or where we might find the info??

Come on guys, don't let the infallability of Locostbuilders down!!!

Probably down to the indicator they use - perhaps this are the points where it changes colour?

Going to google now...

This is the best I can come up with, but I'll warn you I gave up Chemistry as soon as I could and stuck with physics!

- PH neutral is 5.5
- The natural PH of the human body is 7.3
- This would make the values in your question about 1.0 more acidic / alkaline than these so might constitute some sort of safe limits to be handled?

I'm sure someone will give you the real answer tho! Good luck to you both.

D

phenolphthalein changes at 8.3 and methyl orange @ 4.5 ....

clickety click

[Edited on 16/4/09 by stevebubs]

See, told you there'd be a propper answer

D

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Originally posted by stevebubs
phenolphthalein changes at 8.3 and methyl orange @ 4.5 ....

clickety click

[Edited on 16/4/09 by stevebubs]

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second stevebubs, i didnt know the exact change pHs of phenolphthalein and methyl orange but know they are used in A-level chemistry for these exact titrations (or at least they were when i did it!)

another suggestion would be that theyre using EDTA buffer (pH 8.3) also used in A-level chem and bio but not for titrations.

something to do with gobbldygook stuff -

linky if you can follow it

what the feck is acid doing away? dissolving things...is it just breaking bonds between molecules?? How does it do that??????

[Edited on 16/4/09 by Mr Whippy]

Having received an E grade I was surprised I knew this

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Originally posted by damajin
- PH neutral is 5.5

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Originally posted by madmandegge
Having received an E grade I was surprised I knew this

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Originally posted by smart51

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Originally posted by damajin
- PH neutral is 5.5

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I believe it is 7.

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I warned you, I gave up Chem when I was about 13 which was quite a while ago.

D

depressing what you can forget in 15 years

pH is a measure of the concentration of H+ ions in solution; its a logorithmic scale. pH 7 has 1x10 to the power of -7 hydrogen Ions per litre. Alkalis have less Hydrogen ions and more hydroxide ions. Put the 2 together and you get water, hence the "neutral" state at pH7.

Been teaching it all flippin' year for GCSE coursework - HTH!

God A level chemistry, I remember doing that and wishing I hadn't, much harder than maths, the only things I remember was how to name hydrocarbons, free radicals and molar masses!

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Originally posted by tomprescott
God A level chemistry, I remember doing that and wishing I hadn't, much harder than maths, the only things I remember was how to name hydrocarbons, free radicals and molar masses!

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yes maths was easier in that you could work out the answers

whereas with chemistry you had to know so much stuff.

Have forgotten alot of it now tho,

Well done guys! Knew you wouldn't fail!!

Is there a question that someone on here can't answer???

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Originally posted by russbost
Is there a question that someone on here can't answer???

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Originally posted by JoelP
depressing what you can forget in 15 years

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Originally posted by russbost
...My daughter for her coursework has been titrating sodium hydroxide...

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Originally posted by stevebubs

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Originally posted by tomprescott
God A level chemistry, I remember doing that and wishing I hadn't, much harder than maths, the only things I remember was how to name hydrocarbons, free radicals and molar masses!

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Remember the indicators well - mainly because I messed up the titration in my practical (still got a good grade overall, though)

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