The mass of unreacted zinc = 13 g
Further explanation
The reaction equation is the chemical formula of reactants and products
A reaction coefficient is a number in the chemical formula of a substance involved in the reaction equation. The reaction coefficient is useful for equalizing reactants and products
Reaction
[tex]\tt Zn+2HCl\Rightarrow ZnCl_2+H_2[/tex]
mol ratio Zn : HCl = 1 : 2
[tex]\tt mol=V\times M\\\\mol=0.1~L(100~cm^3)\times 2~mol/L\\\\mol=0.2[/tex]
[tex]\tt \dfrac{1}{2}\times 0.2=0.1~mol[/tex]
[tex]\tt 0.1\times 65=6.5~g[/tex]
[tex]\tt 19.5-6.5=\boxed{\bold{13~g}}[/tex]
