What physical feature separates mongolia from china in the southeast?

The answer is the Altai Mountains and the Gobi Desert

China and Mongolia share a large land border stretching to more than 4,700 km
Most of the Western section of this border is covered in the Altai Mountains (Gold Mountains)

In the East, the Gobi desert covers the border, which end in Mongolia and where the grasslands start. 

Mongolia itself is one of the least populated countries in the world. It is the 18th largest country in the world with a population of only 3 million people.

It is also the second largest land-locked country after Kazakhstan