redox and acid

suppose MnO4- is going to react with with Fe2+,we cannot acidified MnO4- by HNO3.My teacher told me that it's because NO3- ion will competee with MnO4- to react with Fe2+.Right? But as I know that, MnO4- is a stronger OA than NO3- wor, then eventually MnO4- will still react with Fe2+ ga,so Why we can't use HNO3 to acidify MnO4-?

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