What is the chamber of the heart the pumps blood into the lungs?

It is the right atrium.

http://www.enchantedlearning.com/subjects/anatomy/heart/labelinterior/glossary.shtml

"The heart is a fist-sized, muscular organ that pumps blood through the body. Oxygen-poor blood enters the right atrium of the heart (via veins called the inferior vena cava and the superior vena cava). The blood is then pumped into the right ventricle and then through the pulmonary artery to the lungs, where the blood is enriched with oxygen (and loses carbon dioxide). The oxygen-rich (oxygenated) blood is then carried back to the left atrium of the heart via the pulmonary vein. The blood is then pumped to the left ventricle, then the blood is pumped through the aorta and to the rest of the body."

http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_anatomy.html

"The ventricle on the right side of your heart pumps blood from your heart to your lungs. When you breathe air in, oxygen passes from your lungs through your blood vessels and into your blood. Carbon dioxide, a waste product, is passed from your blood through blood vessels to your lungs and is removed from your body when you breathe out."

http://kidshealth.org/kid/htbw/heart.html

"Your heart is sort of like a pump, or two pumps in one. The right side of your heart receives blood from the body and pumps it to the lungs. The left side of the heart does the exact opposite: It receives blood from the lungs and pumps it out to the body."