The rules are a bit tricky if you are trying to split one parcel into two. You will need to update us if that is the case.
However, if this is a separate lot and it had a separate purchase price from any other land you owe, your federal capital gains is simply the selling price minus the cost basis. There are certain items you may be able to use to adjust the cost basis to reduce the capital gains, they are listed in IRS publication 523 on page 8 (if though the publication is aimed at home sellers, the same rules apply to vacant land). You pay tax on any capital gains. There is no exemption for small sales (under 25K or any other limit).
https://www.irs.gov/pub/irs-pdf/p523.pdf
