How many liters of chlorine gas at 650 mmhg and 25c can be produced by the reaction of 2.00 l of 2.50 m hcl solution with excess mno2?

First, write the reaction so that you would know the stoichiometric coefficients.

4 HCl + MnO₂ --> Cl₂ + MnCl₂ + 2 H₂O

Moles HCl: 2.50 mol/L * 2 L = 5 mol
Moles Cl₂ produced: 5 mol HCl * 1 mol Cl₂/4 mol HCl = 1.25 mol Cl₂

Assuming ideal gas behavior, 

PV = nRT
Solving for V,
V = nRT/P

The pressure in mmHg is equivalent to atm using the conversion: 760 mmHg = 1 atm.

V = (1.25 mol)(0.0821 L-atm/mol-K)(25+273 K)/(650 mmHg*1atm/760 mmHg)
V = 35.76 L