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A2 chem

help!!!!! very confused i found conc of acid and alkali the took them away leaving acid in excess i dont now why the mark scheme says ka=H+ as i used the ka formla rearranged for H+ by doing ka x HX( where this is mole of acid divide by total volume) to obtain H+ then put H= in log formula

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Reply 1

charco

Study Forum Helper

Original post by Danyal124

After the reaction the concentration of the weak acid, [HA], and the concentration of the salt, [A-], are the same.

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

so if [HA] = [A-]

Then ka = [H+]

Reply 2

Danyal124

Original post by charco

After the reaction the concentration of the weak acid, [HA], and the concentration of the salt, [A-], are the same.

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

so if [HA] = [A-]

Then ka = [H+]

but once the buffer reacts the acid conc goes down to 1.54x10-3 and alkali =0 as its used up. so i dont see why theve used alkali conc in ka formula before bufferreacted and acid in ka formula after buffer reacted

Reply 3

charco

Study Forum Helper

Original post by Danyal124

but once the buffer reacts the acid conc goes down to 1.54x10-3 and alkali =0 as its used up. so i dont see why theve used alkali conc in ka formula before buffer reacted and acid in ka formula after buffer reacted

They are not asking for the pH after the buffer has reacted!

Reply 4

Danyal124

Original post by charco

After the reaction the concentration of the weak acid, [HA], and the concentration of the salt, [A-], are the same.

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

so if [HA] = [A-]

Then ka = [H+]

yes this makes sense but coukld you go in detail about why the ha and a- are then same is it because the buffre stays nutral gets rid of excess acid or alkali