What is the concentration of an HCl solution in the following problem:?

write a balanced equation

Zn(s) + 2HCl(aq) ------------> ZnCl2(aq) + H2(g)

work out moles of Zn

moles = mass / molar mass
moles Zn = 2.0 g / 65.38 g/mol
= 0.03059 moles Zn

Work out moles of HCl in the solution
moles HCl = molarity x Litres
= 6.50 M x 0.0250 L
= 0.1625 moles HCl

According to the balanced equation
1 mole Zn will react with 1 2 moles of HCl
So 0.03059 moles of Zn will react with (2 x 0.03059) moles HCl
= 0.06118 moles HCl

Thus the 2.0 g of Zn will use up 0.06118 moles of HCl

The moles of HCl remaining in solution = total moles HCl - moles HCl used up
= 0.1625 moles - 0.06118 moles
= 0.10132 moles HCl left over

Molarity = moles HCl / Litres solution
M = 0.10132 mole / 0.0250 L
= 4.0528 M
= 4.1 M (2 sig figs)