Answer:
1.5 ml
Explanation:
Assuming that the stomach acid is HCl then:
Mg(OH)₂ + 2HCl → MgCl₂ + H₂O
since
number of moles of Mg(OH)₂ = mass / molecular weight of Mg(OH)₂ = 3*400 mg / 58.3 gr/mol = 20.583 m mol
thus
number of moles of HCl required = number of moles of Mg(OH)₂*2 = 41.166 m mol = 41.166 m moles
knowing that
density = mass / volume = (molecular weight* moles) / volume
volume =(molecular weight* moles)/ density
thus for HCl
volume = (36.46 gr/mol * 41.166*10^-3 moles)/( 1 gr/cm³)= 1.5 cm³= 1.5 ml