In an experimental determination of the universal gas constant R, magnesium reac

Question

In an experimental determination of the universal gas constant R, magnesium reacts with hydrochloric acid and the hydrogen gas is bubbled through water and collected in a tube:

Mg(s) + 2HCl(aq) --> H2(g) +MgCl2 (aq)

During one experiment, 0.150g of Mg produced 209 mL of H2 at 25.4°C when the pressure in the tube was 715 torr. What is the experimental value for R if the vapor pressure of water is 24.4 torr at 25.4°C? (atomic weights: Mg=24.31, H=1.008, Cl=35.45)

a.) o.103 L atm/ mol k

b.) 0.000636 L atm/ mol k

c.) 0.110 L atm/ mol k

d.) 39.222 L atm/ mol k

Explanation / Answer

mol of Mg reacted = mass/molar mass

= 0.150 g/ (24.31 g/mol)

= 6.17*10^-3 mol

mol of H2 formed,

n = mol of Mg reacted

= 6.17*10^-3 mol

p(H2) = 715 torr - 24.4 torr = 690.6 torr = (690.6/760) atm = 0.9087 atm

v = 209 mL = 0.209 L

T = 25.4 oC = (273 + 25.4) K = 298.4 K

use:

p*v = n*R*T

0.9087 atm * 0.209 L = (6.17*10^-3 mol)*R*298.4 K

R = 0.103 atm.L/mol.K

Answer: 0.103 atm.L/mol.K

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