Learn Farsi: پنجمین صندلی برای شما است.

You take the regular number panj (five) and add the suffix -omin. This is the standard way to create this type of ordinal number (like fifth, tenth, etc.) in Farsi.

Why is there no Ezafe connecting "panjomin" (fifth) and "sandali" (chair)?

In Farsi, when you use the -omin type of ordinal number, it always comes before the noun it describes. Because it precedes the noun rather than following it, the Ezafe connector is not used. You simply say panjomin sandali.

Could I say "for you" using a pronoun suffix instead of the separate word "shomā"?

Yes, you could attach the plural/formal pronoun suffix -tān directly to the preposition, making it barāyatān (or barāyatun in spoken Farsi). Using the separate word shomā here is just as natural and can add a slight emphasis to "you".

How would "shomā ast" be pronounced in everyday spoken Farsi?

In casual speech, the verb ast (is) usually drops its "a" sound and attaches to the word right before it. Because shomā ends in a vowel, the combination simply becomes shomāst.