Part 3 Post Module Assessment #19

[SLai_1I]

Does anyone know how to do this question? I've tried 3 times and keep getting 2.45*10^-9 as my answer. Please let me know!

Calculate the reaction quotient, QC, from the following equilibrium data collected in a 3.00 L sealed reaction vessel for the reaction: AsH3 (g) ⇌ As (s) + H2 (g)
AsH3 = 5.55 x 10-4 mol, As = 3.31 x 10-3 mol, H2 = 1.23 x 10-3 mol.

[Arya Adibi 1K]

First, you would have to balance the equation: aAsH3 --> bAs + cH2, where a, b, c are the stoichiometric coefficients.
Then, the expression for Qc would be given by: Qc = [H2]^c/[AsH3]^a

You can calculate the concentrations by taking the mole values given and divide by volume: Conc = n/V

Then it should be straightforward to plug in the values and find the reaction quotient. Not sure if the value you obtained was from this process but I think this should be correct.

[Jay Solanki 3A]

To add, the reason why the As is not included in the equilibrium expression is that it is a pure substance, a solid. Solids do not have any measurable concentration because they are not within a liquid, so they are not included in the equilibrium expression as a measurable concentration amount. Hope this helps!

[Samudrala_Vaishnavi 3A]

Make sure that you divided the mole values of the products and reactants by the 3.00 liters of the vessel mentioned in the problem before you plug it into the icebox. This was a mistake I made the first time. Remember that the concentration is in moles per liter and not just moles. Hopefully, then you can get the right answer!

[SLai_1I]

Ohh ok I see it now. Thanks guys.