Question

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 19.0°C. The volume of the gas is 255 mL measured at 0.971 atm. Calculate the number of grams of sodium used in the reaction. (The vapor pressure of water at 19.0°C = 0.0217 atm.)

Answer 1

pressure of H2 = 0.971 atm - 0.0217 atm

pressure of H2 = 0.9493 atm

now

PV = nRT

Volume of H2 (V) = 255 ml = 0.255 L

Temperature (T) = 19 + 273 K = 292 K

so

0.9493 x 0.255 = nH2 x 0.0821 x 292

nH2 = 0.01

now

consider the given reaction

2 Na + 2 H2O --> 2 NaOH + H2

so

we can see that

moles of Na = 2 times moles of H2

so

moles of Na = 2 x 0.01 = 0.02 mol

now

mass of Na = moles x molar mass

mass of Na = 0.02 mol x 23 g/mol

mass of Na = 0.46 g

so

0.46 grams of sodium is required