This problem is providing information about a reaction between magnesium and hydrochloric acid. However, the moles of the latter are required given 50 cm³ of a 0.2-M solution, making this problem more about the application of molarity. At the end, the answer turns out to be 0.01 mol.
In chemistry, we use units of concentration in order to represent the relative amounts of solute and solvent in a solution. More specifically, molarity relates the moles of the solute with the liters of solution according to:
[tex]M=\frac{n}{V}[/tex]
Thus, since this problem asks for the moles of hydrochloric acid in 50 cm³ of a 0.2-M solution, we must first calculate the volume in liters with:
[tex]V=50cm^3*\frac{1L}{1000cm^3} =0.050L[/tex]
To finally solve for n, moles, to obtain:
[tex]0.2mol/L=\frac{n}{0.05L} \\\\n=0.2mol/L*0.05L\\\\n=0.01mol[/tex]
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