You have a thick Polish accent, not British.
When you are not talking too quickly, I was able to understand, but I've heard a Polish accent before. It's important not to let words run together, especially less common words like "stoichiometry" (which should be 6 syllables in English), and most especially words you expect the listener to be unfamiliar with, like "ChemBuddy".
When you said "millimeters of Hg", it sounded like "mimiters". Slow down and give each syllable its fair share of time. (In British English, certain syllables may be elided, but only in very common word endings that don't really contribute vital information to the meaning).
Another very common error I've seen in many foreign accents is the neglect of syllable length. This can give the impression that you are talking overly fast (and speaking fast and being fluent are two different things). English has both long and short vowels (I mean the actual length of the vowel in time, not the different vowel sounds that we commonly call "long" and "short"). Compare the vowel 'a' in "sat" versus "sad". They are the same vowel sound, but the 'a' in "sad" should be longer. In general, any vowel that appears before a voiced consonant (b, d, g, m, n, ng, r, l) should be long (with some exceptions in multisyllable words).