## IS the pH of PURE WATER always 7? pH of water calculation | A Level Chemistry Tutor

hi I'm a tall and I am in London right

now hi I'm Paul and I'm in Burnley

Lancashire right now okay so we're going

to look at the calculation from the pH

of water and for calculating pH of water

we use the ionic product of water and

that equation is K W equals aqueous

hydrogen ions and aqueous hydroxide ions

and so if we rearrange this for h + KW

over our hydroxide ions and therefore KW

were given in the question

and we're not given hydroxide ions so

you'll notice we don't have enough

information for this in the question we

don't we're not given hydroxide ions so

we're gonna have to think of something

else to do here and what we know with

water what would you know what do we

know about how hydrogen ions and

hydroxide ions in water well yeah for

water there have to be the same amount

to for perfect neutralization you have

to be same so what we can do with water

is safe h+ + rh - ions are equal we can

change this equation to KW equals h plus

squared and therefore if we square root

both sides we get h+ will be equal to

the square root of W and therefore will

be equal to the square root of five

point four eight times 10 to the minus

14 and could you try that in your

calculator yeah just doing that though

my trusty Casio

getting two point three four times ten

to the minus seven so now we've got the

hydrogen ion concentration we can just -

log the answer cuz pH is minus the log

of h plus and that should give us the pH

of our water so to two decimal places

what would that comment us just doing a

minus log the answer quite literally

just there

excellent that gives me 6 point 6 3 1 or

let's call that six point six three six

points if we give ph's to two decimal

places right six point six three it is

the pH of our water no question B says

so why might some students think this

water isn't neutral well I think we've

always been told that pH 7 is water

neutral so this seems to be very

slightly towards the acidic end of the

scale yeah but it's water so we must

know it it's neutral and all that that

pH 7 refers to is the neutrality of

water at 25 degrees C under standard

conditions and here we're not under

standard conditions we're at 50 degrees

C so water at different temperatures

because you get different dissociation

of water at different temperatures you

can get different PHS and so is the

water neutral yes

hydroxide ions still equal the hydrogen

ions of course yeah that's what I was

moment we stopped and they asked me

what's the assumption when neutrality

well hydroxide ions number eight same as

the hydrogen ions so implicit in that

wouldn't be able to do anything unless

he assumed it was neutral exactly we can

only do this calculation because it is

neutral hmm I'm now h+ because I hate

plus c equals our or H my right it's a

great question of just going beyond that

of fixed idea of pH being seven because

I until until now I suppose until I

levels so it's almost like gospel

incidents like page seven days yeah

that's all you've been taught all the

way through school almost yes no thanks

to some advertising by shampoo companies

and soaps as well is like pH heavenly

yes even more regarded as a neutral you

note of course only endure as with many

things in chemistry is that certain

conditions physical conditions

temperature pressure etc so it's a good

go sort of snapping out of that kind of

mode question yeah yeah all right sounds

good now look for to any other of pH

questions here okay

thanks axel there are five huh yeah