I want to apply an algorithm to calculate the clorophyll-a concentration of a reservoir using Sentinel 2.

To do this I need to create a mask using Sentinel's 2 band 11(infrared) to separate the water pixels from pixels with larger reflectance values than the water.

This is the algorithm:

(If B11 <=0.05 then 109.17*((1/B4-1/B5)*B6+ 19.03 else 0)

but I can't use if and else in QGIS.