difference between 'in that case' and 'in which case' ?

[heon]

What's the difference between 'in that case' and 'in which case'?

The following is the(or 'a' I'm not sure about this either) question I found on a Korean English learning website.

Which is correct?
a.Mr. Lambert may retire in May, in that case you will take over his post.
b. Mr. Lambert may retire in May, in which case you will take over his post.

The correct answer was a. according to the website. But there was no clear explanation.

I found another similar question, but this time the answer was different.

a. Some jobs cause clothes to become very dirty, in which case(o) the the company usually supplies uniforms and pays for laundering them.
b. Some jobs cause clothes to become very dirty, in that case(x) the the company usually supplies uniforms and pays for laundering them.

also, no explanation.


I think those two phrases sound slightly different, but that's all. I don't know when to use which.

Please help me.

Thank you.

 

[FreeToyInside]

I don't know whether there's a set rule for the difference between the two, but the difference in the way I always use them is that 'in which case' can link together two clauses in the same sentence, but 'in that case' cannot do this by itself.
In your examples, the way I use these two in AmE makes 'in which case' the correct answer for both.

"It looks like it might snow today, in which case I'll need to wear my big coat."
"It looks like it might snow today. In that case I'll/I'd need to wear my big coat."

However, if I add a conjunction before 'in that case', it feels natural to link two clauses in one sentence.

"It looks like it might snow today, and in that case I'd need to wear my big coat."

As far as beginning a sentence with either, I still get the sense that 'in which case' ties that sentence strongly together with the preceding one.

A: It looks like it might snow today.
B: In which case we'll need to wear our big coats. (This feels like B is tying their sentence to A's sentence, almost like B is finishing A's thought after A has already stopped talking.)

A: It looks like it might snow today.
B: In that case, we'll need to wear our big coats. (This feels like B is referencing A's sentence to make a conclusion or a suggestion, like B is simply stating 'well if that happens/well if that's the case/if it does snow, then we should dress warmer.')


(not a teacher, just a language lover)