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# Calculating H+ from pH and Salt:Acid ratio's watch

1. I never seem to understand how to do either of these. I'll give examples of questions, if someone could be ever so kind in explaining to me how to do them please?

H+ calcuations

Calculate the value of [H+] when the pH of blood is 7.4

Salt:acid

Calculate the ratio of Salt/acid when Ka is 3.9 x 10-8 using

Thanks.
2. (Original post by Loz17)
I never seem to understand how to do either of these. I'll give examples of questions, if someone could be ever so kind in explaining to me how to do them please?

H+ calcuations

Calculate the value of [H+] when the pH of blood is 7.4
.

To calculate H+ you need to do the opposite of working out the pH...so
You do inverse log...comes up with 10^ (10 to the power of) and then the power is -pH

[H+]=10^-7.4
= 3.98x10-8

Get that?? sorry im not so good with the explaining...

I'm just working the other one out!! tell me if you need more explanation!!
3. (Original post by Loz17)
H+ calcuations

Calculate the value of [H+] when the pH of blood is 7.4
Just one formula needed here: pH = - log [H+]

so 7.4 = - log [H+] => [H+]= 10-7.4

Salt:acid

Calculate the ratio of Salt/acid when Ka is 3.9 x 10-8 using

Thanks.

But the ratio is just the fraction {salt/acid}, find that
4. I thought it was someting like that, but I never thought It'd need to be minus pH

Thank you
5. (Original post by EierVonSatan)
Just one formula needed here: pH = - log [H+]

so 7.4 = - log [H+] => [H+]= 10-7.4

But the ratio is just the fraction {salt/acid}, find that
Sorry, both the questions follow on so H+ for salt/acid is what it is in the previous question.
6. (Original post by Loz17)
Sorry, both the questions follow on so H+ for salt/acid is what it is in the previous question.
Ahh okay so then:

salt/acid = (3.9 x 10-8)/(10-7.4)
7. Ahh i remember the days when this stuff was new and difficult.
8. Ahh ok, so its just the answer of then?
9. (Original post by vas876)
Ahh i remember the days when this stuff was new and difficult.
the worrying thing is that I don;t think I was ever taught it, and this is for my self taught retake!
10. (Original post by Loz17)
Ahh ok, so its just the answer of then?
Say you get a value of 7.9 for salt/acid...

this means there is 7.9 molecules of salt for every 1 acid molecule (i.e. 7.9 = 7.9/1)
11. Ahhh ok I get it

Thank you all
12. (Original post by Loz17)
the worrying thing is that I don;t think I was ever taught it, and this is for my self taught retake!
I self taught it, the first time.

Luckily im retaking CHM5 and not CHM4, but i still remember how to do it, its literally plugging numbers into the formula.
13. (Original post by vas876)
I self taught it, the first time.

Luckily im retaking CHM5 and not CHM4, but i still remember how to do it, its literally plugging numbers into the formula.
The rest of pH calculations are just plugging numbers in, I didn't think these would be different but I had no idea on these two.

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