What is the cause of this "white out" on the right side of the chest x-ray?

Pleural effusion.  Click image to enlarge.

CT of same patient demonstrating left pleural effusion.

A couple features suggest pleural effusion over other causes:

• homogeneous opacity with no air bronchograms
• concave upper surface producing a meniscus 
• while not really notable with this pleural effusion, some large pleural effusions will displace the mediastinum towards the contralateral side 

The "white out" is unlikely to be caused by lobar collapse because there is no evidence to suggest decreased lung volume (displacement of pulmonary fissure, elevation of hemidiaphragm, displacement of hila, displacement of mediastinum or compensatory overinflation of an adjacent lobe).   Collapse of each lobe has a characteristic radiographic appearance, click here to view.

Pneumonia is also an unlikely cause.  For one, there is a second area of consolidation at the left lung base (also a pleural effusion).  While multifocal pneumonia does exist, non-infective pathology takes a higher place in the differential when there are multiple areas of consolidation.  Additionally, pneumonia generally has a more inhomogeneous opacification with air bronchograms and ill-defined margins. Click here to view a large pneumonia.


Source

Jenkins, P.  Making Sense of the Chest X-Ray: A Hands-On Guide, 1st ed.

Lisle, D. Imaging for Students, 3rd ed.

Schwartz, D.  Emergency Radiology: Case Studies