What is the correct pronunciation of 唯一

Question

For the word (or composition of two words): 唯一, sometimes also written as: 唯一つ,

Two possible pronunciations come to my mind:

• ゆいいつ [yuiitsu]　-onyomi

and

• ただ ひとつ [tada hitotsu] -kunyomi

Which one is correct? Does the last つ in written form make any difference?

Answer 1

唯一 and 唯一つ both work as a no-adjective and a standalone adverb, but they are different.

• 唯一 is one word and only read as ゆいいつ in modern Japanese. (ゆいいち is another possible reading according to this entry, but I believe you can forget it)
• 唯一つ (with okurigana つ) is two words, ただひとつ.

They are not always interchangeable. For example you can say 唯一の生存者 ("sole survivor") and 唯【ただ】一人【ひとり】の生存者 but not 唯一つの生存者.