Charles law gives the relationship between temperature of gas and volume of gas.
It states that for a fixed amount of gas, temperature is directly proportional to volume of gas.
V / T = k
where V- volume , T - temperature and k - constant

parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation.
T1 = 250 °C + 273 = 523 K
T2 = 150 °C + 273 = 423 K
Substituting the values in the equation,

V = 251 mL
the new volume is 251 mL
Use PV = nRT
(2 atm)(.3 liters) = n(8.314 mol*K)(303°K)
.6 = n(2519.142)
Divide by 2519.142
n = .00023818 mols of HCl * 36.46g of HCl/ 1 mol of HCl
Grams of HCl = 0.00868
Answer: E. It decreases the value of Eact.
Explanation:
Activation energy is the extra energy that must be supplied to reactants in order to cross the energy barrier and thus convert to products.
A catalyst is a substance which increases the rate of a reaction by taking the reaction through a different path which involves lower activation energy and thus more molecules can cross the energy barrier and more molecules convert to products.
The catalyst itself does not take part in the chemical reaction and is regenerated as such at the end.