gains on land sale and purchase

[Sam Brown]

Hi,

I have a primary residence on a large piece of land. I'm selling a lot from it (say for $300k) and at the same time buying an adjacent lot (say for $100k). 

How does capital gains work in this case? Can I claim the sale was for the difference ($300k-$100k=$200k) and then only pay gains based upon that figure?

If I sell additional lots, how do the lots get treated? Any specific rules since they are "part" of the primary residence.

thx