In a sentence: I use a(n) hydrometer to measure the concentration of a solution..... The "a" seems to be correct but "an" sounds correct. Which should it be?
hi.
there is nothing to be confused
we just use "an" for the nouns that start with these letters:(a,e,o,i,u) and we use "a" for the rest of nouns.
examples:
an orange/an umbrella/an elephant/an apple/an idiot person/
a sheep/a telephone ....
When it comes to words starting with a spoken "h" sound, such as in the original question, there is a (not often observed) rule as follows:
Before words where the stress falls on the first syllable (including single-syllable words), a is correct, never an:
A hit; a whole lot; A history of Britain; a handy short-cut.
Before words where the stress falls on a syllable other than the first one, an is correct, though a is widely used:
An hotel; an historic occasion; an horologist
This is so little understood and inconsistently observed that you can get away with using a everywhere except where the "h" is silent (e.g. an honourable man; an honest mistake). But to be absolutely correct and have the satisfaction of smiling inwardly at everybody else's puzzled looks , just follow the above guidelines. Good luck!