Question

What is the percentage yield of 10.1g mg reacts with an excess of water and 21.0 g Mg(OH)2 is removed ?

Answer 1

Here is a worked example of a stoichiometry problem. Follow that and find g Mg(OH)2 that is the theoretical yield.. Then
%yield = [actual g Mg(OH)2/theoretical yield)*100 = ?
http://www.jiskha.com/science/chemistry/stoichiometry.html