An antacid contains 200 mg of aluminum hydroxide and 200 mg of magnesium hydroxide per capsule. how many moles of aluminum hydroxide does each capsule contain?

200 mg in g:

200 mg / 1000 => 0.2 g

number of moles Al(OH)3 : 

molar mass = 78.00 g/mol

1 mole 78.00 g
? moles 0.2 g

0.2 x 1 / 78.00 =>  0.00256 moles of Al(OH)3 

number of moles Mg(OH)2 :

molar mass = 58.32 g/mol

1 mole 58.32 g
? moles 0.2 g

0.2 x 1 / 58.32 => 0.00343 moles of Mg(OH)2 

hope this helps!