Which one is the correct way of writing:
- The sugar concentration was 20 g/mL or
- The sugar concentration was 20 g/ mL
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g/mL or g/ml
The International Standard allows L as an alternative to the canonical l and different national bodies and style guides differ as to whether they recommend, allow, prohibit or require the capital L, so if you have a style-guide you are writing to then use it, otherwise use L with American English (to match NIST guidelines) and l otherwise.
Space within a unit like that means multiplication (as does the clearer mid-dot ·). 20 g/ml being the same as 20 kg/l is hence the same as 20 kg l-1 or 20 kg·l-1 but a space after a solidus makes no sense.
(You might for typographical reasons use a very thin space like a hair-space in the final lay-out if it aids appearance, but such spaces are so thin that they don't look like spaces so much as like slightly different kerning).