Chromium metal reacts with acid to produce Cr3+ ions and hydrogen gas. — Gases Chemistry Question
Problem Context
Chromium metal reacts with acid to produce Cr3+ ions and hydrogen gas.
Write a balanced equation for this reaction.
Model Answer
2Cr + 6H+ → 2Cr3+ + 3H2
When a sample of chromium metal is reacted with excess acid, 94.7 mL of gas is collected over water at 745 mm Hg and 20˚C. Assuming ideal gas behavior determine the mass of metal reacted. (The vapor pressure of water at 20˚C is 24 mmHg.)
Model Answer
The pressure of H2(g) produced is: 745 mmHg – 24 mmHg = 721 mmHg
Determine number of moles of hydrogen:
= 0.00373 mol H2
Now get mass of chromium:
= 0.129 g Cr
State and explain how the volume of gas would change if Cr2+ (rather than Cr3+) ions were formed in this reaction.
Model Answer
The volume of gas would be only 2/3 as much (63.1 mL) becase each Cr atom would release only two electrons to reduce the H+ ions (rather than 3.)
Determine the number of molecules of water vapor that would be present in the volume of gas produced assuming ideal behavior.
Model Answer
= 1.24 × 10^-4 mol H2
and
= 7.49 × 10^19 molecules H2
Calculate the ratio of the average molecular velocity of hydrogen to the average molecular velocity of water vapor at the same temperature.
Model Answer
= 3 so the ratio of velocities is 3:1
The van der Waals equation for real gases is
The coefficients, a and b, for the hydrogen gas are a = 0.242 atm·L2·mol–2 and b = 0.0266 L·mol–1. The corresponding values of a and b for sulfur dioxide are 6.714 and 0.05636, respectively.
i. Identify the molecular property that corresponds to the a coefficient and account qualitatively for the difference between its values for these two gases.
Model Answer
The “a” coefficient is part of the term that is a correction factor for the attractive forces between molecules. SO2 has a larger value for “a” because SO2 molecules have stronger forces (due to it being both larger than H2, and polar.)
ii. Identify the molecular property that corresponds to the b coefficient and account qualitatively for the difference between its values for these two gases.
Model Answer
The “b” coefficient is part of the term that is a correction factor for the molecular volume. Because SO2 is a larger molecule, (more volume) the value for “b” is larger.