Question

Chemistry 1215, Experiment #11; Determination of the Atomic Weight of an Unknown Metal, Pre-lab Name ____________________________________ 1. Based on equation 2 in the introduction, to what group in the periodic table does the unknown metal probably belong? Explain your rationale. 2. Explain why it is necessary to consider water vapor pressure in calculating the atomic mass of the unknown metal. Would the calculated atomic mass be larger or smaller than the true value if water vapor pressure is ignored? 3. A 40.0 mg sample of a metal reacts with HCl according to the equation in the text. 24.4 mL of gas at 20.0 o C is collected by the downward displacement of aqueous acid as described in the lab protocol. If the atmospheric pressure is 780 mm Hg, what is the atomic weight of the metal? 4. A student allowed several air bubbles to enter his eudiometer tube. He ignored the error and proceeded with the experiment. What effect will this error have on his final result? Explain your rationale.

Answer 1

For the reaction,

1. The metal belongs to alkaline earth metal series

2. water vapor pressure of water varies with temperature and we are measuring volume of gas with water displacement. So water vapor pressure hanges would affect the result.

3. Using,

moles = PV/RT

           = ((780-17.5)/760) x 0.0244/0.08205 x (273+20)

           = 0.001 mols

atomic weight of metal = 0.04/0.001 = 40 g/mol

4. If air was introduced into the system, the collected volume of gas would change and this would give a higher volume of gas. The calculated molar mass would be lower.