The following reaction is used to generate hydrogen gas in the laboratory. If 243 mL of gas is collected at 25 °C and has a total pressure of 0.9933 bar, what mass of hydrogen is produced? A possibly useful table of water vapour pressures is provided below. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) T (°C) P (bar)
20 0.023398
25 0.031704
30 0.042477

Question

Quizplus · Accepted Answer

First, we need to calculate the partial pressure of hydrogen using Dalton's law of partial pressures: 
P(H2) = P(total) - P(H2O)

From the table, at 25°C, the water vapor pressure is 0.031704 bar. 
Thus, P(H2) = 0.9933 bar - 0.031704 bar = 0.961596 bar

Next, we can use the ideal gas law to find the number of moles of hydrogen produced: 
PV = nRT

n = PV/RT = (0.961596 bar)(0.243 L)/(0.0821 L·atm/mol·K)(298 K) 
n = 0.00968 mol H2

Finally, we can use the molar mass of hydrogen to find the mass produced: 
mass = n·Molar mass = 0.00968 mol · 2.016 g/mol 
mass = 0.0195 g H2

Therefore, the best choice is C as it is closest to this value.

The Following Reaction Is Used to Generate Hydrogen Gas in the Laboratory