AQA A-level Chemistry 7405 - Paper 3 - 19th June 2019 unofficial mark scheme

For the Ka question I'm not convinced you guys have the right answer. Usually we're supposed to assume concentration of HX is constant whereas some of it will react to form H+/X-. It usually changes so little that the concentration is the same to 3sf. However in this case this was not a valid assumption as HSO4- was actually quite a strong acid and the actual concentration was not the same as the original concentration to 1sf even. Not only that but if you take this into account usually when finding ph this is quite tough but in this case it was very straightforward as you knew the concentration of H+ already.

I don’t think it was -0.8V it was positive cause 0.8+0.33=1.13

Why are you overcomplicating a simple 6 marker?

isn't the % uncertainty ＝0.005/0.685 something

Cause I think that that is the right thing to do. Also I wasn't overcomplicating it, since its all in a fixed volume to find the new [HSO4-] you just subtract the [H] from the original one. So much of that HSO4- is going to dissociate that I don't see how it can be right not to take it into account.

I got 1.47 too

It was once used to measure mass of container and once to measure mass of container and solid. Uncertainty will be ×2

But doesn’t 0.005/0.685 * 100 (%) get you like 0.7%?

Yh that’s right but multiply by 2

One of the assumptions for weak acids is that it doesn’t dissociate much. So HSO4- initial = HSO4- equilibrium

'A weak acid is an acid chemical which does not dissociate (split into ions) completely in water solution. This means it does not give all its hydrogen ions into the water. Weak acids typically have a pH between 3 and 6.' There is nothing about assuming [HX] being constant in that definition. The fact that the PH was like 1.4 or something showed it wasn't a typical weak acid and that assumption is just not valid. Usually if you work it out the concentrations are the same to 3sf but in this case they weren't to 1sf.

Everyone saying salt bridge for the question asking what is A. They asked for the APPARATUS, not what it is. So it's a U - TUBE. the next question you could say use it as a salt bridge

Everyone saying salt bridge for the question asking what is A. They asked for the APPARATUS, not what it is. So it's a U - TUBE. the next question you could say use it as a salt bridge

I thought the same as you but thought that since it was a 6 marker and not worth more marks (as it wouldn't be in an A2 question anyway) I just assumed that [HA] remained constant, so did mass/volume. I think you're right though - [HA] would decrease so you would expect a larger Ka value. The real standard Ka value for HSO4^- is apparently 1.2*10^-2moldm^-3 which is larger than 7*10^-3mol^dm^-3 that was calculated so this matches up with the previous statement

Thanks! Yeah I got 1.2*10^-2.

Thanks! Yeah I got 1.2*10^-2.