when you bought the property you bought the building and land. based on the acquisition cost at that time you would need to allocate the total cost between land and building. then you would need to know the square footage of the land. unless you can justify a different allocation, you would allocate the cost of the land sold
based on the ratio of sq footage sold to total sq footage.
there is a provision in the code to use the home sale exclusion for the gain on the sale of the vacant land but that requires the residence be sold within two years of the sale of the land - (reg 1.121-1(b)
also you must have owned the property and occupied the residence for at least any 2 years out of up to 5 years before sale, if married you would each be entitled to the exclusion if only one of you own the property for that period but both need to have occupied it for the 2 year period. next the home sale exclusion hasn't been used in the 2 years before this sale on a previous residence.
you can elect not to use the gain exclusion if you qualify by simply reporting the sale on form 8949
