Question

when 2.4 g zinc is added to hydrochloric acid, 450 ml of hydrogen gas form at a temperature of 32 C what is the pressure of the gas

Answer 1

First we need to find the number of moles of hydrogen gas formed
Zn + 2HCl ---> ZnCl₂ + H₂
stoichiometry of Zn to H₂ is 1:1
the number of Zn moles reacted - 2.4 g / 65.4 g/mol = 0.0367 mol
assuming Zn to be the limiting reactant 
number of Zn moles reacted = number of H₂ moles formed 
therefore number of H₂ moles formed = 0.0367 mol

we can use ideal gas law equation to find the pressure
PV = nRT
P - pressure
V - Volume - 450 x 10⁻⁶ m³
n - number of moles - 0.0367 mol
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - temperature - 32 °C + 273 = 305 K
substituting these values in the equation 
P x 450 x 10⁻⁶ m³ = 0.0367 mol x 8.314 Jmol⁻¹K⁻¹ x 305 K
P = 206.8 kPa 
pressure is 206.8 kPa