You are Here: Home

# pH and Kw calculations help. watch

1. Ok.. so to start with, define the term Kw.. all I could find was its an equilibrium constant?

1) calculate the pH of water at 50 oC. Explain why pure water at 50 oC is still neutral even it's pH is not 7.

calculation -

explanation -

Don't get this at all...

2) At 25 oC, Kw has the value 1.00 x10-14.mol2dm-6. calculate the pH at 25 oC of
i)0.150moldm-3 solution of sodium hydroxide.

ii) the solution formed when 35.0cm3 of this solution of NaOH is mixed with 40.0cm3 of a 0.120moldm-3 solution of HCl.

3) In a 0.150moldm-3 solution of a weak acid, HX at 25 oC, 1.80% of the acid molecules are dissociated into ions.

i) write an expression for Ka for the acid HX.

ii) Calculate the value of Ka for the acid HX at this temp and state its units.
2. For a start Kw is the equilibrium constant for water
2H2O -> H3O+ + OH-
This is temperature dependant. Kw is different at 50C because more dissociates, this affects the H+ concentration since it effectively splits into H+ and OH- and pH is a measure of H+ concentration
3. (Original post by bex_1_2_1)
Ok.. so to start with, define the term Kw.. all I could find was its an equilibrium constant?

1) calculate the pH of water at 50 oC. Explain why pure water at 50 oC is still neutral even it's pH is not 7.

calculation -

explanation -

Don't get this at all...

2) At 25 oC, Kw has the value 1.00 x10-14.mol2dm-6. calculate the pH at 25 oC of
i)0.150moldm-3 solution of sodium hydroxide.

ii) the solution formed when 35.0cm3 of this solution of NaOH is mixed with 40.0cm3 of a 0.120moldm-3 solution of HCl.

3) In a 0.150moldm-3 solution of a weak acid, HX at 25 oC, 1.80% of the acid molecules are dissociated into ions.

i) write an expression for Ka for the acid HX.

ii) Calculate the value of Ka for the acid HX at this temp and state its units.
It is not possible to calculate the pH at 50ºC without further information.

It is true that:

kw = [H+][OH-]

and that kw = 1 x 10-14 @ 25ºC

BUT without knowing exactly how changing the temperature affects the dissociation quantitatively there is no way to calculate the new pH...

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 25, 2011
Today on TSR

### Top unis in Clearing

Tons of places at all these high-ranking unis

### Why do women like men in suits?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE