Use this guide to upgrade the RAM in your Mid/Late-2011 21.5" iMac to a maximum of 16GB (8GB x2 - i3 model), or 32GB (8GB x4 - i5/i7 models).
Note: The i3 model (late 2011) 21.5-inch iMac is a special release to educational institutions only. Unlike other iMac12,1 models, the i3 model just 2 memory sockets, and is limited to a total of 16GB RAM. The specific model number is A1311 and EMC 2496.
Parts