# Quick titration question

#### Tokspor

Let's say there are 50.0 mL of an unknown concentration of acid which definitely only has one proton. It is titrated with 50.0 mL of a 0.10 M NaOH. By using M1V1 = M2V2, it looks to me like the concentration of the unknown acid would be 0.10 M.

My question is, is this 0.10 M H3O+ or 0.10 M HA?

#### crazybob

it's 0.10 M HA. you would need the K a of the acid to calculate [H3O+]

OP
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#### Tokspor

Thanks crazybob. Am I right in thinking that the reason why it's 0.10 M HA is because while the -OH will deprotonate all the HA, we don't know to what extent HA deprotonates by itself since it could be a weak acid?

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#### crazybob

Thanks crazybob. Am I right in thinking that the reason why it's 0.10 M HA is because while the -OH will deprotonate all the HA, we don't know to what extent HA deprotonates by itself since it could be a weak acid?
yes. if HA dissociated completely, then you would have 0.10 M H3O+ and that would be neutralized completely by 0.10 NaOH.

the Ka value allows us to see how well the HA dissociates into A- and H+ (or H3O+ in water)