Question

in the reaction Mg (s) + 2HCl (aq) -> H2 (g) + MGCl (aq), how many moles of hydrogen gas will be produced from 75.0 milliliters of 1.0 M HCL in an access of Mg?

Answer 1

Step One
Find the moles of HCl
C = 1.0 mol/L
V = 75 mL = 75 / 1000 = 0.75 L 
Note the concentration is in Liters.

Formula
n = C*V 

Solve
n = 1.0 * 0.075 
n = 0.075 moles

Step two 
Adjust the number of moles of HCl
How every mol of H2 produced, you will require 2 mol of HCl

$\frac{2 mol HCl}{1 mol H_2} = \frac{0.075\text{mol of HCl given}}{x}$

2/1 = 0.075/x  Cross multiply
2x = 0.075      Divide by 2
x = 0.075 / 2
x = 0.0375      moles of H2  <<<<<<<<< Answer