Question

DETERMINATION OF THE MASS OF MAGNESIUM METAL IN A SAMPLE EXPERIMENT 12 PRE-LAB QUESTIONS Fully answer these questions in your laboratory notebook before coming to lab. Show all work for numerical calculations. 1. How many moles of hydrogen are present in a 43.50 mL sample at 25.5 °C and 772.8 torr? (assume the hydrogen is dry) 2. What is the mass of a piece of magnesium wire which produces 38.25 mL of hydrogen gas collected over water at 22.0 °C and at a pressure of 756 torr? 3. You collected a sample of hydrogen gas in an inverted buret by displacement of water at 23.5°C. The buret could not be submerged deep enough in the water bath to equalize pressure. The water level in the buret was 9.35 cm above the water level in the water bath. The volume of gas in the buret was determined to be 40.55 mL. a. If the current atmospheric pressure was 29.14 in of Hg, what is the pressure of dry hydrogen in the buret? Show work. b. How many moles of hydrogen are in this sample? Show work. SAMP

Answer 1

1) The moles are calculated:

n = P * V / R * T = (772.8 / 760) atm * 0.0435 L / 0.082 * (25.5 + 273) K = 0.0018 mol

2) The vapor pressure of the water at that temperature is 19.8 Torr, the pressure of dry H2 is calculated:

P H2 = 756 - 19.8 = 736.2 Torr

The moles of gas are calculated:

n H2 = (736.2 / 760) * 0.03825 / 0.082 * (22 + 273) = 0.0015 mol

The magnesium mass is calculated:

m Mg = n * MM = 0.0015 * 24.3 = 0.036 g

3) Water column pressure, water vapor pressure and atmospheric pressure in Torr are calculated:

P column = 9.35 cm H2O * (1 Torr / 1.36 cm H2O) = 6.88 Torr

P vap = 21.6 Torr

P atm = 29.14 in Hg * (25.4 Torr / 1 in Hg) = 740.2 Torr

a) The H2 pressure is calculated:

P H2 = P atm + P column - P vap = 740.2 + 6.88 - 21.6 = 725.48 torr

b) The moles of H2 are calculated:

n H2 = (725.48 / 760) * 0.04055 / 0.082 * (23.5 + 273) = 0.0016 mol

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