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QLDChemistryQuick questions
Unit 2: Molecular interactions and reactions
Quick questions on Stoichiometry of reactions involving gases (QCE Chemistry Unit 2)
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the four-step mole map?Show answer
For any stoichiometric problem:
What is gas-volume ratios at constant T and P (Avogadro's law shortcut)?Show answer
When all species are gases and all at the same temperature and pressure, the volume ratio of any two species equals the mole ratio. You do not need to convert through moles at all.
What is worked example?Show answer
Calculate the volume of CO_2 produced at SLC when 12.0 g of calcium carbonate decomposes completely.
What is limiting reactant problems with gases?Show answer
When two reactants are given (one or both gases), determine the limiting reactant first.
What is percent yield with gases?Show answer
Percent yield = (actual yield / theoretical yield) x 100 percent. Express both yields in the same units (both as volume, both as mass, or both as moles).
What is including condensed-phase species in a volume ratio?Show answer
Only gases (g) participate. Liquid water or solid products do not occupy gas volume at the stated conditions.
What is using V_m at non-standard conditions?Show answer
V_m = 24.79 L/mol applies only at SLC. For other conditions, use PV = nRT to find moles, do not assume the same V_m.
What is forgetting to identify the limiting reactant?Show answer
With two given amounts you must check ratios. If you assume both fully react, you will overestimate the product.
What is dropping the gas-state label?Show answer
Reactions like Mg + HCl produce H_2(g); the gas-volume question only makes sense for the gas product, not for MgCl_2(aq).
What is confusing mass yield and volume yield?Show answer
Percent yield is one ratio (actual / theoretical) in any consistent unit. Mixing mass for one and volume for the other gives nonsense.