URGENT help needed pleeease! any Maths /chemists/physists?? help with a caculation!

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emzie
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#1
Report Thread starter 16 years ago
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hiya can someone plz help! I have to work out the concentration of ethanoic acid from the pH of the solution, and the concentration of the acids in vinegar from the pH of the solution. I have a worked example:

Ka = 1.74 x 10-5 pH = 2.55

[CH3COOH(aq)] = [H+(aq)]2 / Ka


[H+] = pH
-log10[H+] = 2.55
log10[H+] = 2.55
[H+] = 2.82 x 10-3 mol dm-3
Ka = 1.74 x 10-5 mol dm-3
Therefore, [CH3COOH(aq)] = (2.82 x 10-3)2 / 1.74 x 10-5
= 0.457 mol dm-3


but how do I get from the log10[H+] = 2.55 to [H+] = 2.82 x10-3 ?

PLLLLEEEAASSSE please please someone help! I've been wraking my brains for hours and it has to be in tomorrow and I stil have the rest of my analysis and evaluation to go (i've been working solidly on it for the past 3 wks!-my cwk that is!) please someone help!
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elpaw
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(Original post by emzie)
hiya can someone plz help! I have to work out the concentration of ethanoic acid from the pH of the solution, and the concentration of the acids in vinegar from the pH of the solution. I have a worked example:

Ka = 1.74 x 10-5 pH = 2.55

[CH3COOH(aq)] = [H+(aq)]2 / Ka


[H+] = pH
-log10[H+] = 2.55
log10[H+] = 2.55
[H+] = 2.82 x 10-3 mol dm-3
Ka = 1.74 x 10-5 mol dm-3
Therefore, [CH3COOH(aq)] = (2.82 x 10-3)2 / 1.74 x 10-5
= 0.457 mol dm-3


but how do I get from the log10[H+] = 2.55 to [H+] = 2.82 x10-3 ?

PLLLLEEEAASSSE please please someone help! I've been wraking my brains for hours and it has to be in tomorrow and I stil have the rest of my analysis and evaluation to go (i've been working solidly on it for the past 3 wks!-my cwk that is!) please someone help!
2.82*10^-3 = 10^-(2.55)
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emzie
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Report Thread starter 16 years ago
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(Original post by elpaw)
2.82*10^-3 = 10^-(2.55)

thanks does ^ mean x10 to the power of? coz i did that in my calculator and it said 'syn error'??

i typed in 10 EXP -(2.55) and it said syn error? As you can see im not mathmatically minded!
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meepmeep
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If you tap in 1 EXP -2.55 you will get what you want and ^ simply means to the power of, so x10^ means times ten to the power of.
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elpaw
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(Original post by emzie)
thanks does ^ mean x10 to the power of? coz i did that in my calculator and it said 'syn error'??

i typed in 10 EXP -(2.55) and it said syn error? As you can see im not mathmatically minded!
syn error means youre putting it in the wrong order/using the wrong buttons. did you use the take-away '-'? you have to use the minus '-'
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piggsy_123
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(Original post by emzie)
thanks does ^ mean x10 to the power of? coz i did that in my calculator and it said 'syn error'??

i typed in 10 EXP -(2.55) and it said syn error? As you can see im not mathmatically minded!
^ means "to the power of"

not "x10 to the power of"
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emzie
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ahhhhh thanks guys yeh piggsy i was doing x10 to the power of instead of to the power of! oooppsss! thnx loads!!! bloomin eck tho i'm gunna be up all nyt - literally! o well my dad gave me loads of red bull! lol
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