Chemistry question

[Krissy]

How to do you find the pH of a solution when you're given the concentration of it?

and how would you do this question:

You wish to prepare a solution of barium hydroxide with a pH of 13.500. What mass of the compound be required to dissolve in 500mL of water?

My teacher never really explained this :/

[`Roze`]

Oh jeez. We did these in chemistry a few weeks ago. But I didn't quite understand it myself. 

/isnohelp

[Harrstein]

hmmm if i find some time i think i'll try getting the log shit together again

[Harrstein]

ok if my memory serves me right (only doing strong bases and acids because with weak ones i'd really have to think)

the pH of a acid solution has all to do with the amount of H⁺ ions, an acid basically sheds those H⁺ ions. it doesnt love them and therefor kicks them out of the home. then a caring H₂O molecule passes by and adopts the H⁺, creating a H₃o⁺ ion.

with  strong acids it is fairly easy. 1 acid sheds 1 H⁺ ion, so 1 mole of acid gives us 1 mole of H⁺ and with that 1 mole of H₃o⁺

the first step would be to convert the amount of moles in a solution to a concentration. say i have 0,175 mole of HCl in 2L of water, this would be 0,875 mole/L

because HCl is a strong acid we can say that therefor there are 0,0875 mole/L H₃O⁺ ions in our solution.

we throw this trough a simple formula
pH = - log₁₀( H₃O⁺ )
if we fill this in we get
pH = - log₁₀( 0,0875 )
we throw this in our trusty calculator and we'll get something in the likes of
ph = 1,06


for base this is kinda the same but with different ions
now we use OH^- instead of H₃o⁺
with this you calculate
pOH = - log₁₀( OH^- )
keep in mind i said pOH, to convert that to pH you simply do
pH = 14 - pOH

2nd question of you will follow later



Keep in mind that i havent even thought about chemistry in half a year and that my memory on such things can differ a bit from reality.
if someone could confirm or reject what i said above i'd appreciate it

also i dont think it is safe for me to answer stuff about weak bases and acids as that is long ago for me, at least 2 years ago

[Krissy]

Thanks! I got second question already (: