DAT Destroyer Gen Chem question on PH Estimating

Mrhyde

Becoming Dr. Jekyll
DAT Destroyer for the Gen Chem section number 247

My question is, that i could just use logic without having to do a lot of math to get this answer correct but it did not come out correct so i want to know what is flawed ?

HBr is a strong acid and we have 6 * 10 ^-4

KOH is a strong base and we have 12 * 10 ^-4

----- Even though this has nothing to do with titrations this was my thinking: From my titration notes I know that at the equivalence point of a Strong acid and a Strong base (which is the point we have equal amount of acid and equal amounts of base) the PH will be 7.

So then I thought that here we have acid and base at a ratio of 6 :12 which breaks down to a 1:2 ratio. So we have two times as much base. So I know the answer must be basic, but now the question is by approximately how much? Then I though that since the ratio is basically a 1:2 ration then we should have the ph increase into basic conditions by only one factor of 10 which would put the ph at around 8 for the final answer.

But damn I was wrong the answer is 12. My logic came from my notes in my Buffer and Titration chapters, even though I know this is not a buffer but a strong base and a strong acid = 7 in a titration at a 1:1 ratio should still be true but then why would it not be an increase by 1 factor of 10. why does this not work here

Crithu

2+ Year Member
DAT Destroyer for the Gen Chem section number 247

My question is, that i could just use logic without having to do a lot of math to get this answer correct but it did not come out correct so i want to know what is flawed ?

HBr is a strong acid and we have 6 * 10 ^-4

KOH is a strong base and we have 12 * 10 ^-4

----- Even though this has nothing to do with titrations this was my thinking: From my titration notes I know that at the equivalence point of a Strong acid and a Strong base (which is the point we have equal amount of acid and equal amounts of base) the PH will be 7.

So then I thought that here we have acid and base at a ratio of 6 :12 which breaks down to a 1:2 ratio. So we have two times as much base. So I know the answer must be basic, but now the question is by approximately how much? Then I though that since the ratio is basically a 1:2 ration then we should have the ph increase into basic conditions by only one factor of 10 which would put the ph at around 8 for the final answer.

But damn I was wrong the answer is 12. My logic came from my notes in my Buffer and Titration chapters, even though I know this is not a buffer but a strong base and a strong acid = 7 in a titration at a 1:1 ratio should still be true but then why would it not be an increase by 1 factor of 10. why does this not work here
It doesn't look like you took into account the volume of solution anywhere. When calculating pH and pOH, you need to include volume because 1 mole of base in 1 liter of water is not the same as 1 mole of base in 10 liters of water. The only thing the ratio will tell you is how much of the Base will react with the Acid. Once you have left over base, you need to put that over volume to get concentration.

Then you can plug it in to pOH=-log[OH] and then pH=14-pOH