pH (1 Viewer)

[blam_babe]

How do I work out the pH of a solution when it's basic ie has OH- rather than H+ ions floating around???

I know it has something to do with the number 14...

 

[xiao1985]

yea... usually, at room temp/ pressure, - log {[h] x [oh]} = 14 or soemthing... i forgot... but to calculate it, calculate pOH den take it away from 14... tis pH...

 

[+:: $i[Q]u3 ::+]

[OH-][H+] = 10^-14
i.e. the concentration of hydroxide ions x the concentration of hydrogen ions = 10 to the power of -14.

or as xiao said, u can calculate pOH
pOH = -log [OH-]
and pH = 14 - pOH.

 

[blam_babe]

ok i did that and i got a pH of 17.653.....
agh
i need help
ok the qn is :
Find the pH pf a solution of magnesium hydroxide of concentration 4.5 x 10^-3

 

[+:: $i[Q]u3 ::+]

lol... well i chucked out my calculator ages ago so this has no numbers =P

magnesium hydroxide = Mg(OH)2.
so concentration of hydroxide ions [OH-] = 2 x 4.5 x 10^-3
= 9.0 x 10^-3
Now pOH = - log [OH-]
= - log [9.0 x 10^-3]
now when you've done that... say u get an answer n.

pH = 14 - n.

check ur doing log 10, not natural log. um.. make sure ur signs are correct... u should get an answer greater than 7 yeah?

 

[blam_babe]

ok i did it again and got 11.954... which sounds about right - i guess.
another qn - where does the 4.5 from

concentration of hydroxide ions [OH-] = 2 x 4.5 x 10^-3

come from???

 

[+:: $i[Q]u3 ::+]

Originally posted by blam_babe
Find the pH pf a solution of magnesium hydroxide of concentration 4.5 x 10^-3

lol u said it ^^

 

[blam_babe]

Hahaha ok

Lol im going insane :s

 

[abdooooo!!!]

Originally posted by blam_babe
Hahaha ok

Lol im going insane :s

hahaha... you're funny

 

[blam_babe]

hahaha... you're funny

funny but its sadly true

 

[CM_Tutor]

Another approach is to say that pH = 14 + log[OH-], where log is log base 10.

 

[blam_babe]

Another approach is to say that pH = 14 + log[OH-], where log is log base 10.

Yeah I think that this is the way I was taught it in class, but we basically did a lesson on how to do the [H+] only, not the [OH-] or when it's basic.

Which method would be better in an exam?? Like the non C1V1=C2V2 one is better than C1V1=C2V2. So should I use the above stated one or:

magnesium hydroxide = Mg(OH)2.
so concentration of hydroxide ions [OH-] = 2 x 4.5 x 10^-3
= 9.0 x 10^-3
Now pOH = - log [OH-]
= - log [9.0 x 10^-3]

????

 

[CM_Tutor]

Either is fine, as both are valid - in fact, they are the same method, just done in slightly different ways mathematically. The real 'trick' (if you can call it that) in this question is noting that the formula of magnesium hydroxide is Mg(OH)2, and thus that [OH-] = 2 * [Mg(OH)2].