real quick, can someone explain please?? (1 Viewer)

[abc123doremi]

bored seems to be running short of members right now

i can't get my head around this ...either something i don't know or i've just removed the very brick that will cause the foundation of chemical chemistry to collapse in on itself...though i doubt the latter

consider this problem:
find the pH of a 4.0 x 10^-13 mol/L solution of NaOH (assuming it completely dissociates)

this means that [Na⁺] = [OH^-] = 4.0 x 10^-13

to find pH, we find pOH:
pOH = -log (4.0 x 10^-13)
= 12.4

now ionisation/water constant
pK_W = pH + pOH = 14

therefore
pH = 14 - 12.4 = 1.6

that is really acidic, even though i only added a base

in short: what the hell?

i'm thinkin le chatelier reversing the changes or whatever, but like, that pH is even lower than hydrochloric acid pH.

 

[dp624]

If you actually had a solution of pH 1.6
then the concentration of OH- would be 4.0 x 10^-13
remember that at neutral the concentration of OH- is 1x10^-7

but, if you had a neutral solution
then added NaOH to make it 4.0x10^-13 molar, you'd be adding like micrograms or so.
And it'd be pH 7

don't worry, im sure you wont get this question, it seems really retarded

 

[abc123doremi]

i still don't understand
you're not adding micrograms, you're just adding NaOH solution of really low concentration
why doesn't the maths check out
like from pH 7 to 1.6 just by adding base and not acid

 

[dp624]

no, cos you see
when you're dealing with such low concentrations
then the latent (already-present) NaOH in the water makes a HUGE impact. remember that water dissociates naturally.

normally the [oh-] is 10^-7. when you add 4x10^-13 [naoh], then the resultant [oh-] is 10^-7.

 

[abc123doremi]

why is the resultant pOH 7?
do you like multiply it or add it or what?

 

[dp624]

you have to add them

but trust me you won't get this sorta question.
like ever

 

[abc123doremi]

k thanks heaps brother

 

[Alchemist K]

i still don't understand
you're not adding micrograms, you're just adding NaOH solution of really low concentration
why doesn't the maths check out
like from pH 7 to 1.6 just by adding base and not acid

The maths do check out, its only because the HSC doesn't stress the maths behind your problem unless you do Industrial or something where you learn about equilibrium constants and the like. What the other guy said is pretty much correct but its not a stupid question and was actually raised during a lecture.

1) In 'normal HSC' circumstances when we calculate the pH or pOH of solutions the OH and H concentrations are at levels higher than 10^-7 which we can take to be the concentration of H+ at 25C. In other words, the H+ from the water plays a small part in determining the overall concentration of H+, think about adding 0.000000005 to 0.5.

2) In circumstances you mentioned where the amount of H+ added is small compared to the amount of H+ already present then the small amount added plays a minimal part like adding 0.00000000005 to 0.5 except that 0.5 is now the H+ in water.

3) You're unlikely to be asked to calculate this in the HSC but you MAY be asked to reason out and explain a likely pH.