Question

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A piece of sodium metal reacts completely with water as follows:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
The hydrogen gas generated is collected over water at 25.0°C. The volume of the gas is 307 mL measured at 0.975 atm. Calculate the number of grams of sodium used in the reaction. (The vapor pressure of water at 25.0°C = 0.0313 atm.)

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Answer 1

volume = 307 mL = 0.307 L

Temeprature = 25 + 273 = 298 K

pressure = 0.975 - 0.0313 = 0.9437 atm

P V = n R T

0.9437 x 0.307 = n x 0.0821 x 298

n = 0.01184

moles of H2 gas = 0.01184

2 mole Na-----------------> 1 mol H2

x mol Na --------------> 0.01184 mol H2

x = 2 x 0.01184 = 0.02368

moles of Na = 0.02368

mass of Na = moles x molar mass

                  = 0.02368 x 23

                  = 0.545 g

mass of Na = 0.545 g