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QLDChemistryQuick questions
Unit 2: Molecular interactions and reactions
Quick questions on Concentration and dilution of aqueous solutions (QCE Chemistry Unit 2)
12short 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 standard concentration units?Show answer
For dilute aqueous solutions where density is approximately 1.00 g/mL:
What is calculating molarity?Show answer
where c is in mol/L, n in mol, V in L. Rearrangements:
What is worked example?Show answer
A chemist needs 250.0 mL of 0.0500 mol/L silver nitrate solution.
What is interconverting units?Show answer
Mass concentration to molarity:
What is dilution?Show answer
Diluting a solution increases the volume but does not change the amount of solute. Therefore concentration falls in inverse proportion to volume.
What is solution stoichiometry?Show answer
Solution stoichiometry uses the same mole map as gas stoichiometry, with V x c replacing V_m or PV/(RT) at the solution end.
What is serial dilution?Show answer
A serial dilution is a sequence of dilutions, each one applied to the previous result. The total dilution factor is the product of the individual dilution factors.
What is mixing volume units?Show answer
mL and L must be consistent. Divide mL by 1000 to get L before using in c = n / V.
What is adding solvent to the wrong volume?Show answer
Volumetric flask procedure is "make up to" the mark, not "add" that volume of water. Final volume is the marked volume of the flask, not the starting volume plus added water.
What is using density 1.00 g/mL for concentrated solutions?Show answer
The water approximation breaks down above about 10 percent w/v. For dilute environmental and biological work it is fine.
What is ppm vs ppb vs %?Show answer
Order of magnitude: 1 percent = 10,000 ppm = 10,000,000 ppb. Misreading by a factor of 1000 is a common single-mark loss.
What is forgetting that dilution does not change n_solute?Show answer
The number of moles of solute in stock equals the number of moles in the diluted aliquot used; only the volume of solvent surrounding it has changed.