Archipelago is pronounced ar-kə-PEL-ə-go (IPA /ˌɑːrkɪˈpɛləɡoʊ/). The 'ch' is hard /k/; stress the third syllable 'PEL'.
IPA: /ˌɑːrkɪˈpɛləɡoʊ/
Syllables: ar · kə · PEL · ə · go
Common ESL mistake: Learners pronounce 'ch' as /tʃ/ saying 'ar-chip-el-uh-go'.
Tip: The 'ch' is hard /k/; stress the third syllable 'PEL'.