A reaction is 2 H2 + O2 -> 2 H2O. If 4.0 g of H2 reacts completely with excess O2, what mass of H2O is produced? Show the worked solution.

When 4.0 g of H2 reacts completely with excess O2, 36 g of H2O is produced.

Worked Solution:

1. Calculate the molar masses:

   • Molar mass of H2 = 2 * (1.008 g/mol) = 2.016 g/mol
   • Molar mass of H2O = (2 * 1.008 g/mol) + (15.999 g/mol) = 18.015 g/mol

2. Convert the mass of H2 to moles: Moles of H2 = Mass of H2 / Molar mass of H2 Moles of H2 = 4.0 g / 2.016 g/mol = 1.984 mol H2

3. Use the mole ratio from the balanced equation to find moles of H2O: The balanced equation is 2 H2 + O2 -> 2 H2O. From the equation, 2 moles of H2 produce 2 moles of H2O. The mole ratio is 1:1. Moles of H2O = Moles of H2 * (2 mol H2O / 2 mol H2) Moles of H2O = 1.984 mol H2 * (1 mol H2O / 1 mol H2) = 1.984 mol H2O

4. Convert moles of H2O to mass: Mass of H2O = Moles of H2O * Molar mass of H2O Mass of H2O = 1.984 mol * 18.015 g/mol = 35.744 g

5. Round to appropriate significant figures: The given mass (4.0 g H2) has two significant figures. Therefore, the answer should also be rounded to two significant figures. Mass of H2O = 36 g