# Acid and Base equillibrium Flashcards

Define, by the bronsted and lowry definition, what is meant by an acid.

- a substance that donates H+ ions

Define, by the bronsted and lowry definition, what is meant by a base

- a substance that accepts H+ ions

in which case will you decide whether it is an acid or alkali?

- the one that has a higher Ka value

give the formula for pH

pH= -log[H+]

give the formula to find the concentration of H+ ions

- 1x10^pH

how to calculate Kw

= [H+] [OH-]

what is the value of Kw at 25 degrees c for most solutions? give units

1x10^-14 mol^2dm^-6

what does pKw mean?

-log[Kw], so the value would be -log1x10^-14, which would

= 14.

How to find pH of pure water?

- Kw^2 = H+ concentration
- so use this to find the pH

why does water’s pH change at temperatures different to 25 degrees?

- Le Chatelier’s

principle can predict the change. - dissociation of water is endothermic so

increasing the temperature would push the

equilibrium to the right

giving a bigger

concentration of H+ ions

resulting in a lower pH.

How do we work out the pH of a strong base?

- rearrange Kw expression to find H+ ions (so Kw/OH-)
- find pH using log expression

what is the expression for the weak acid dissociation expression?

Ka= [H+][A-]/ [HA]

what does a larger Ka value signify?

- that the acid/base is stronger

what 2 assumptions are made when calculating pH of a weak acid?

- initial conc of the undissociated acid stays constant, as it’s very small, so eq HA = Initial HA
- [H+] = [A-] , as the dissociation has taken place in a 1:1 ratio.

taking into account these assumptions, what does the Ka expression simplify to?

Ka= [H+]^2/[HA] initial

What needs to happen in order for us to assume that pKa=pH?

- when a weak acid has reacted with exactly half the neutralisation volume of alkali, so half the point of neutralisation.

how to work out the pH of a diluted acid?

H+ (diluted) = old vol/new vol x H+ (old)

- then use the log expression to work out pH