Chemistry - Mass of Iron

Question

A sample of iron was dissolved in hydrochloric acid. The liberated hydrogen was collected over water. The hydrogen occupied 262.0 mL at 26.0°C. The barometer pressure was 748 mm Hg. What mass of iron was dissolved? The vapor pressure of water vapor at 26.0°C is 25.2 mm Hg.

Answer 1

Fe(s) + 2HCl(aq) ==> H₂(g) + FeCl₂(aq) ... balanced equation

Given:

262.0 ml of H₂ gas formed at 26.0ºC (26 + 273 = 299K)

atmospheric pressure = 748 mm Hg and vapor pressure of H₂O @ 26ºC = 25.2 mm Hg

Asked for:

mass of Fe dissolved

Approach:

... using the ideal gas law (PV = nRT) find the moles of H₂ formed. To do this, we must convert temperature from C to K and we must subtract the vapor pressure of H₂O from the atmospheric pressure to obtain the pressure from H₂ gas only. We should also use the appropriate value of R (gas constant) which is 62.36 L-mm Hg/K-mol) to be consistent with the units of pressure.

... from moles of H₂ gas, we can find moles of Fe by using the stoichiometric ratios in the balanced equation.

... from moles of Fe, we an convert to mass of Fe using the atomic mass of Fe (55.85 g/mole).

PV = nRT and solve for n

n = PV/RT = (748 mm - 25.2 mm)(0.262 L)/(62.36 L-mm Hg/K-mol)(299K) = (722.8)(0.262)/(62.36)(299)

n = moles H₂ = 0.01016 moles

moles Fe = 0.01016 moles H₂ x 1 mole Fe/mole H₂ = 0.01016 moles Fe

mass Fe = 0.01016 moles Fe x 55.85 g/mole = 0.567 g Fe (to 3 significant figures)