Topic 5.8-5.18 - Quantitative analysis

Question 1

How do you calculate the concentration of solutions in mol dm–3 ?

Answer 1

Concentration​ ​of​ ​a​ ​solution​ ​can​ ​be​ ​measured​ ​in​ ​moles​ ​per​ ​given​ ​volume​ ​of solution​ ​e.g.​ ​moles​ ​per​ ​dm3​ ​​ ​(mol/dm3​ )​

To​ ​calculate​ ​moles​ ​of​ ​solute​ ​in​ ​a​ ​given​ ​volume​ ​of​ ​a​ ​known​ ​concentration​ ​use
moles​ ​=​ ​conc​ ​x​ ​vol​ ​i.e.​ ​mol​ ​=​ ​mol/dm3​ ​​ ​x​ ​dm​3​​

Question 2

How do you convert concentration in g dm–3 into mol dm–3 and vice versa?

Answer 2

g​ ​dm​-3​​ ​->​ ​mol​ ​dm​-3​​ ​divide​ ​by​ ​molar​ ​mass/relative formula mass ​in​ ​grams
mol​ ​dm​ ​ ​→​ ​g​ ​dm​ multiply​ ​by​ ​molar​ ​mass/relative formula mass ​in​ ​grams

Question 3

Describe how to carry out an accurate acid-alkali titration, using burette, pipette and a suitable indicator

Answer 3

○ add​ ​acid​ ​to​ ​burette​ ​using​ ​a​ ​funnel,​ ​record​ ​the​ ​volume​ ​in​ ​the​ ​burette​ ​to start
○ add​ ​known​ ​volume​ ​of​ ​alkali​ ​to​ ​a​ ​conical​ ​flask​ ​and​ ​add​ ​some​ ​indicator
○ place​ ​conical​ ​flask​ ​on​ ​white​ ​tile​ ​(so​ ​you​ ​can​ ​see​ ​colour​ ​change​ ​clearly)
○ add​ ​acid​ ​to​ ​alkali​ ​until​ ​you​ ​reach​ ​the​ ​end​ ​point
○ calculate​ ​how​ ​much​ ​acid​ ​has​ ​been​ ​added​ ​(titre)
○ repeat​ ​until​ ​you​ ​get​ ​concordant​ ​titres

Question 4

How would you calculate an unknown concentration of a solution using the results of a titration?

Answer 4

if​ ​you​ ​only​ ​knew​ ​the​ ​concentration​ ​of​ ​the​ ​acid​ ​and​ ​wanted​ ​to​ ​calculate​ ​the concentration​ ​of​ ​the​ ​alkali:
○ calculate​ ​moles​ ​of​ ​acid​ ​using​ ​moles​ ​=​ ​concentration​ ​x​ ​volume
○ calculate​ ​the​ ​mole​ ​ratio​ ​of​ ​acid​ ​to​ ​alkali​ ​using​ ​the​ ​equation​ ​for​ ​the reaction
○ work​ ​out​ ​how​ ​many​ ​moles​ ​of​ ​alkali​ ​you​ ​have​ ​using​ ​the​ ​mole​ ​ratio​ ​and moles​ ​of​ ​acid​ ​(e.g.​ ​if​ ​you​ ​have​ ​5​ ​moles​ ​of​ ​acid​ ​and​ ​the​ ​ratio​ ​of​ ​acid​ ​to alkali​ ​is​ ​1:2,​ ​you​ ​will​ ​have​ ​10​ ​moles​ ​of​ ​alkali)
○ calculate​ ​the​ ​concentration​ ​of​ ​the​ ​alkali​ ​using​ ​concentration​ ​=​ ​mol​ ​/ volume

Question 5

What is the calculation for percentage yield?

Answer 5

Percentage yield = actual yield/theoretical yield x 100

Question 6

Why is the actual yield of a reaction usually less than the theoretical yield?

Answer 6

Due to:
* Incomplete reactions
* Practical losses during the experiment
* Competing, unwanted reactions (side reactions)

Question 7

What is the calculation for atom economy?

Answer 7

atom​ ​economy​ ​=​ ​(Mr​ ​of​ ​desired​ ​product​ ​from​ ​reaction​ ​/​ ​sum​ ​of​ ​Mr​ ​of​ ​all reactants)​ ​x​ ​100

Mr = relative formula mass

Question 8

Why may a particular reaction pathway be chosen to produce a product?

Answer 8

Based on whether it has high atom economy, high yield, fast rate, equilibrium position to the right and useful by-products

Question 9

What is the molar volume?

Answer 9

The molar volume, of any gas at room temperature and pressure, is the volume occupied by one mole of molecules of any gas at room temperature and pressure

Question 10

How can you use a balanced equation with a molar volume to calculate the mass of a solid?

Answer 10

○ calculate​ ​moles​ ​of​ ​the​ ​reactant
if​ ​given​ ​a​ ​volume:​ ​moles​ ​=​ ​volume​ ​÷​ ​24
● work​ ​out​ ​the​ ​mole​ ​ratio​ ​and​ ​so​ ​work​ ​out​ ​how​ ​many​ ​moles​ ​of​ ​the​ ​product you​ ​have
● calculate​ ​mass ​using​ ​moles:
mass=moles​ ​x​ ​molar​ ​mass

Question 11

How can you use a balanced equation with a molar volume to calculate the volume of a gas?

Answer 11

○ calculate​ ​moles​ ​of​ ​the​ ​reactant
if​ ​given​ ​a​ ​mass:​ ​moles=mass​ ​÷​ ​formula​ ​mass
if​ ​given​ ​a​ ​volume:​ ​moles​ ​=​ ​volume​ ​÷​ ​24
● work​ ​out​ ​the​ ​mole​ ​ratio​ ​and​ ​so​ ​work​ ​out​ ​how​ ​many​ ​moles​ ​of​ ​the​ ​product you​ ​have
● calculate​ ​volume​ ​using​ ​moles
for​ ​calculating​ ​volume:
volume=moles​ ​x​ ​24

Question 12

How would you use Avogadro’s law to calculate the volume of a gas?

Answer 12

Avagadro’s law (6.02 x 10^23) x the number of moles