Question

Question 2 (0.5 marks) Hydrogen gas, H2, and oxygen gas, Oz, can be reacted to form liquid water. If 2.00 g hydrogen gas and 17.20 g oxygen gas are reacted in a 1.500 L flask at 25.0 °C, calculate the total gas pressure (atm) in the flask at 25 °C. The vapor pressure of water is 23.8 mm Hg. Stoichiometry is involved in this question. Total gas pressure:

Answer 1

Step 1: Calculate the number of moles of hydrogen and oxygen.

Number of moles of hydrogen = Given mass of hydrogen/molecular mass of hydrogen

Number of moles of hydrogen = 2/2 =1mole.

Number of moles of oxygen = 17.20/32 = 0.5375 mole.

Step 2: Finding the limiting reagent.

Balanced equation for the reaction of hydrogen and oxygen is

2H₂ + O_{​​​​​​2} -------> 2H₂O

According to the given balanced equation

2 moles of hydrogen can react with 1 mole of oxygen, then

1 mole of hydrogen can react with, number of moles oxygen...

Number of moles of oxygen = 1/2 =0.5 moles.

This, suggests that oxygen is available in excess, and hence hydrogen is the limiting reagent.

Step 3: Finding the number of moles of un reacted oxygen

Number of moles of oxygen consumed in the reaction is

= 0.5 mole.

Number of moles of un reacted oxygen = 0.5375 - 0.5

= 0.0375 mole.

Step 4: Calculation of the pressure of unreacted oxygen

According to Ideal gas equation

PV = nRT (R = 62.39 L.mm/k.mol; T = 25 + 273 = 298)

P = nRT/V

P = 0.0375 X 62.39 X 298 / 1.5

P = 464.8055 mm of Hg.

Step 5: Calculation of total pressure

Total pressure = Pressure of oxygen + Vapour pressure of water

= 464.805 + 23.8

= 488.60 mm Hg.