Question

a 0.784 g sample of magnesium is added to a 250 ml flask and dissolved in 150ml of water. magnesium hydroxide obtained from the reaction required 215.0 mL of 0.300 M hydrochloric acid to completely react. How many moles of Mg were used?

Answer 1

The chemical reaction involving Mg(OH)2 and HCl is:

Mg(OH)2  +  2HCl  -->  MgCl2  +  2H2O

 

So we see that for every 2 moles of HCl, 1 mole of Mg is reacted.

Calculating for moles HCl:

moles HCl = 0.300 M * 0.215 L

moles HCl = 0.0645 mol

 

The moles Mg then is:

moles Mg = 0.0645 mol * (1 / 2)

moles Mg = 0.03225 mol