Name: Section: Determination of Gas Constant Date: Post-Lab Questions n evaluati

Question

Name: Section: Determination of Gas Constant Date: Post-Lab Questions n evaluation of R was performed using the procedures outlined in this exercise. The barometric pressure was 735 torr and the temperature was 21.5 °C· The volume of H2(g) collected was 32.7 mL. The calculated value of R was 0.0817L atm/mol-K. 1. A (a) How many grams of Mg metal were used in this determination? (b) If the correction for the vapor pressure of water had not been performed, what would be the calculated value of R? (c) If the syringe volume had been incorrectly read resulting in a calculated H2 volume of 30.6 mL, what would be the percent error in the calculated value of R?

Explanation / Answer

1. evaluation of R by experiment

(a) moles of Mg = PV/nR

with,

P = (735 - 19.2)torr/760 = 0.941 atm

V = 0.0327 L

R = gas constant

T = 21.5oC + 273 = 294.5 K

So,

moles Mg = 0.941 x 0.0327/0.0821 x 294.5 = 0.0013 mol

mass of Mg = 0.0013 mol x 24.305 g/mol = 0.0316 g

(b) If the correction of vapor pressure of water is not done, the final pressure would be greater than the actual value. So the R calculated would also be greater than the actual value. R and P are directly proportional.

PV = nRT

(c) If the volume of H2 was incorrectly read to a lower volume. The

the value of R thus calculated would be,

R = 30.6 x 0.0817/32.7 = 0.0764 L.atm/K.mol

percent error = (0.0817 - 0.0764) x 100/0.0817 = 6.5%

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