Can you tap a land for mana and then float that mana before playing the land counts as your land drop?

A land must be on the battlefield before it can be tapped for mana. Cards in your hand have no activated abilities and cannot be tapped. The act of 'playing a land' means moving it from your hand directly onto the battlefield as a special action — it does not pass through the stack (CR 305.1). Only once it resolves and enters the battlefield does it become a permanent you can tap.

Tapping a permanent for mana is an activated ability (written as 'Tap: Add [mana]'). Activated abilities can only be activated while the permanent is on the battlefield, unless the card specifically says otherwise (CR 602.1, CR 109.2). There is no game rule that lets you access a land card's mana ability from your hand.

Mana floating (having mana in your mana pool at the end of a step or phase) is a real concept under CR 500.4, but it is irrelevant here because you can never generate the mana from a land before it enters the battlefield in the first place.

Concrete example: You have a Forest in hand and want to cast Giant Growth (G) this turn. You play the Forest as your land drop — it enters the battlefield. Then you tap it for G, which floats until you spend it on Giant Growth. You cannot tap the Forest while it is still in your hand to 'pre-float' the mana before playing it.