A.)Magnesium, an alkaline earth metal reacts with water producing hydrogen gas.

Question

A.)Magnesium, an alkaline earth metal reacts with water producing hydrogen gas. If one decides to collect the hydrogen over water at 20 degrees celsius and the total amount of gas collected is 0.639 L with pressure of 1.11 atm, what is the amount of hydrogen in g in the sample? Vapor pressure of water at 20 degrees celsius is 17.5 torr.

Balance equation

Mg(s)+ 2H2O(l)-> Mg(OH)2(aq) + H2(g). THe mass of hydrogen collected over water is?

B.) Magnesium carbonate can decompose into magnesium oxide and carbon dioxide.

MgCO3(s)-> MgO(s)+ CO2(g)

If the has collected over water at 10 degrees celsius has a total pressure of 1.23 atm and volume of 1.563 L, how many grams of magnesium carbonate was decomposed? the vapor pressure of water at 10 degrees celsius is 9.2 torr.

Explanation / Answer

(A) Mg(s) + 2 H2O(l) => Mg(OH)2(aq) + H2(g)

P(H2) = P(total) - P(H2O)

= 1.11 - 17.5/760 = 1.087 atm

V = 0.639 L, T = 20 deg C = 293.15 K, R = 0.08206 atm.L/mol.K


Ideal gas equation: PV = nRT

Moles of H2 = n = PV/RT

= 1.087 x 0.639/(0.08206 x 293.15) = 0.028874 mol


Mass of H2 = moles x molar mass of H2

= 0.028874 x 2.016

= 0.0582 g


(B) MgCO3(s) => MgO(s) + CO2(g)

P(CO2) = P(total) - P(H2O)

= 1.23 - 9.2/760 = 1.218 atm

V = 1.563 L, T = 10 deg C = 283.15 K, R = 0.08206 atm.L/mol.K


Ideal gas equation: PV = nRT

Moles of MgCO3 = Moles of CO2 = n = PV/RT

= 1.218 x 1.563/(0.08206 x 283.15) = 0.08193 mol


Mass of MgCO3 = moles x molar mass of MgCO3

= 0.08193 x 84.314

= 6.91 g

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