The Ionic Product For Water

C h e m g u i d e – q u e s t i o n s

THE IONIC PRODUCT FOR WATER, K

1. a) Write the expression for K

b) What are the units for K

c) At a temperature a tiny fraction of a degree less than 25°C, the value for K

is 1.00 x 10

-14

(in the

units you will have quoted in your last answer). Use this information to show why the pH of pure

water is 7 at this temperature.

-14

d) At 30°C, K

has a value of 1.471 x 10

(in the same units). Calculate the pH of pure water at

this temperature. Is the water now acidic, alkaline, or still neutral? Explain your answer.

e) As the temperature increases, the value of K

increases. Use your knowledge of Le Chatelier's

Principle to work out whether the ionisation of water is exothermic or endothermic:

2. a) Define pK

b) What is the value of pK

at 30°C? (Use the K

value from Q1(d).)

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