The new purchase doesn't matter.
If you made more than a 250,000 (500,000 for joint) gain then the amount over it is taxed. Doesn't matter what you did with the proceeds like buy another house or pay off the mortgage. The rule about rolling over the gain to the next house went out in May 1997. But if you rolled over a gain before then it will decrease your cost basis when you sell after May 1997.
See publication 523 Selling your home
https://www.irs.gov/pub/irs-pdf/p523.pdf
Enter a primary house sale under
Federal Taxes
Wages and Income
Choose Explore on my own or I'll choose what to work on (if it comes up)
Then scroll down to the last section - Less Common Income
Sale of Home - Click the Start or Update button
