meaning - "With the purpose of" vs. "With the intention of"

A private student of mine had to complete a gap fill text, which contained the following excerpt:

Pronunciation isn't my strongest point, therefore I've decided to improve it.
I've borrowed some Spanish DVDs from the library, with the purpose
of listening to native speakers and trying to copy them.

The correct answers, according to Cambridge English Complete CAE, were so and intention. Although I would have written so because a comma preceded the gap, and explained this to the Italian student (who rightly huffed) I could not explain why the noun, intention, was preferred. To me with the purpose of and with the intention of are synonymous.

If the phrase had been written as follows:

I've borrowed some Spanish DVDs from the library for the purpose
of listening to native speakers and trying to copy them.

Would purpose have been more appropriate here? Should there be a comma after library? I ask because in the text above there is a comma preceding with (I loathe having to explain punctuation, but seeing as it's towards exam preparation I'd like to be as thorough as possible).

P.S. The exercise was not a multiple choice one, the learner has to supply the one word answer that best fits.

EDIT There is indeed an "and" in the last sentence, which I missed when writing the excerpt. Many thanks to Edwin Ashworth and @DavidSchwartz for pointing out the (mea culpa) transcription error.

Answer

Pronunciation isn't my strongest point _ __ _ I've decided to improve
it. I've borrowed some Spanish DVDs from the library, with the _ __ _
of listening to native speakers and trying to copy them.

The first gap (in my version) would require so or ,so or ;therefore . Given the comma, 'therefore' is ungrammatical as it never follows a comma.

The second gap could certainly be filled by intention. 'I've borrowed some Spanish DVDs from the library, with the intention of listening to native speakers and trying to copy them.' is a paraphrase of the more colloquial 'I've borrowed some Spanish DVDs from the library. I thought I'd listen to some native speakers and try to copy them.'

It could also be filled by purpose. However, construction-wise, 'for the purpose of' is more idiomatic than 'with the purpose of'. And semantically (and this is probably why the preferred choice of preposition is as it is), there is more of a flavour of the ultimate achievement than need be present with 'with the intention of' and certainly 'I thought I'd'. This doesn't sit too well just before 'listening to'. 'For the purpose of' would sit better with 'really getting to grips with idiomatic Spanish' (Aim, less 'ultimate achievement'-flavoured would sit happily with both 'listening' (ie the strategy employed) and proficiency (ie 'copying' – speaking like – 'the native speakers': the ultimate achievement)).