# pH equivalence

• Feb 3rd 2008, 07:57 AM
johett
pH equivalence
50mL of 0.10mol/L hydrobromic acid is titrated with 0.10mol/l aqueous ammonia. Determine the pH at equivalence.

NH4+H2O -->NH3+H3O

n=c x v
=0.10mol/L x 0.05L
=0.005mol

V=0.005mol
0.1mol/L
=0.05L

Vtotal=0.1L

[NH4Br]=0.005mol
0.1L
=0.05mol/L

NH4 + H2O --> NH3 + H3O
0.05 0 0
-x +x +x
0.05-x x x

Ka= Kw
1.8 x10^-5
=5.6x10^-10=\$\displaystyle x^2\$/0.05
x=5.27x10^-6

pH=5.28

I understand everything but how do u know that the Vtotal is 0.1L???
• Feb 3rd 2008, 08:04 AM
bobak

and can you at try and make your working a little more clear. I can make out that "n = c x v" is means "moles is concentration times volume" but i cant follow the rest of it.
• Feb 3rd 2008, 08:06 AM
johett
sorry about the first post everyone...I hope the one I have now is a bit easier to understand. :)
• Feb 3rd 2008, 08:21 AM
bobak
the stiochemistry gives that away.
• Feb 3rd 2008, 08:42 AM
johett
also, how do u know that you're suppose to use the 0.1L and not 0.05L from the previous step to answer the step after the Vtotal?
• Feb 3rd 2008, 02:23 PM
bobak
you need to look at the stiochemistry of the overall reaction, at equivalence you need to have one mole of the titre for ever mole of the substance your titrating. because they are both of the same concentration you will need the exact same volume of each to get the same number of mole of both of them.

in simple terms 0.05 + 0.05 = 0.1

had the concentration of the ammonia been 0.05mol/L you would have needed twice the volume hence the total volume in that case would be 0.05 + 0.1 = 0.15.

are you following ?