Question

Zinc and magnesium metal each react with hydrochloric acid to make chloride salts of the respective metals, and hydrogen gas. A 10.00 g mixture of zinc and magnesium produces 4.157 L of hydrogen gas at 1.589 atm and 27.0*C upon being mixed with an excess of hydrochloric acid.

a. Determine the percent magnesium by mass in the original mixture.

Answer 1

a)

% Mg by mass

% Mg = mass of Mg / total mass * 100%

m = 10 g of sample

mass of zinc = 10-x

mass of Mg = x

then:

mol of Mg = x/MW iof Mg = x/24.3

mol of Zn = (10-x) / (65.4)

get moles of H2:

mol =

Apply Ideal Gas Law,

PV = nRT

where

P = absolute pressure

V = total volume of gas

n = moles of gas

T = absolute Temperature

R = ideal gas constant

n = PV/(RT) = 1.589*4.157/(0.082*300) = 0.268515 moles of H2

0.268515 mol = mol of Zn + mol of Mg

0.268515 = (10-x) / (65.4) + x/24.3

0.268515*24.3*65.4 = (10-x)*24.3 + 65.4*x

426.729 = 243 - 24.3x+ 65.4*x

426.729 -243  = 41.1x

x = 183.729/41.1 = 4.4702

mass of Mg = 4.4702 g

mass of Zn = 10-4.4702 = 5.5298 g

% mass of Mg = 4.4702 /10*100 = 44.7 %