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?

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