A0.784 g sample of magnesium is added to a 250 ml flask and dissolved in 150 ml 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 hcl were used?

A lot of values are given but we must focus only on HCl.

There are 215 mL (0.215 L) of 0.300 M HCl that was completely   used. We must remember the moles is simply the product of volume and molarity, that is:

number of moles = 0.215 L * 0.300 M

number of moles= 0.0645 moles HCl