Is zakat obligatory on land that the mother bought with the intention of building, then left for sale after the son bought a better plot of land?

The land your mother bought for the purpose of building, and which you later intended to sell, is not subject to zakat (definition). This is because its purchase was for acquisition (qinya) and not for trade. This is the view of the majority of jurists, such as Al-Shafi'i, Abu Hanifa, Malik, and Al-Thawri.