Chemical analysis

Question 1

Gravimetric analysis steps

Answer 1

🔹 Weighing sample to be analysed
🔹 Stirring sample into solvent
🔹 Filtering off undissolved solids
🔹 Adding excess reagent to precipitate with part of substance
🔹 Filtering, drying and weighing the precipitate

Question 2

What does gravimetric analysis assume

Answer 2

🔹 Complete dissolving of all the substance being analysed for
🔹 None of the substance remains with the filtered undissolved solids
🔹 All of the substance that can precipitate, does so
🔹 Nothing else in the solution precipitates with the excess reagent
🔹 The precipitate is pure and completely dry before weighing

Question 3

Error: if BaCl2 is not added in excess to precipitate the Sulfur in the fertiliser

Answer 3

🔹 Not all sulfate ions in the solution will precipitate to form ba2So4
🔹 Mass of BaSo4 in the crucible is lower, lower mole ratio
🔹 Mole of Sulfur is too low
🔹 Percentage of sulphur is too low

Question 4

Errors leading to high mass of precipitate

Answer 4

1) Did not dry the precipitate completely.
2) Did not wash the precipitate with water to remove water soluble species such as those used to cause the precipitation.
3) Incorrectly weighed sample (e.g. fertilizer) and recorded a lower mass value than the real value - leads to a higher %w/w.

Question 5

Errors leading to lower mass of precipitate and percentage of Sulfur

Answer 5

1) Incomplete precipitation reaction due to reasons such as not adding enough precipitating agent, not waiting long enough for reaction completion, precipitation performed at a low temperature (slow rate of reaction).
2) Not cooling reaction before filtration of precipitate as at higher temperature the precipitate will give better solubility.
3) Incomplete collection of precipitate due to reasons such as spilling precipitate or not completely collecting precipitate.
4) Incorrectly weighed sample (e.g. fertilizer) and recorded a higher mass value than the real value - leads to a lower %w/w.

Question 6

Standard solution

Answer 6

Solution of accurately known concentration

Question 7

Primary standard

Answer 7

A substance for which an accurately known amount can be dissolved to make an accurately known volume of solution to determine an accurate concentration

Question 8

What must a primary standard be

Answer 8

🔹 Pure and have a known formula (with high relative mass)
🔹 Be soluble, inexpensive and have a high relative mass
🔹 Not change composition during storage

Question 9

Why are NaOH and HCl not suitable primary standards

Answer 9

🔹 NaOH reacts with Co2 and absorbs water from the atmosphere
🔹 Pure HCl is a gas, difficult to completely dissolve a known mass

These substances would need to be made to an approximate concentration and then standardised using a titration (secondary standard solutions)

Question 10

Aliquot

Answer 10

Accurate volume of a solution delivered by a pipette

Question 11

Equivalence point

Answer 11

The point in a titration where there is no excess of either reactant according to the stoichiometry of the equation

Question 12

Endpoint

Answer 12

The point in a titration when there is a permanent colour change

Question 13

Burette

Answer 13

Delivers an accurate volume (titre)

Rinse with solution it is to deliver

Question 14

Volumetric flask

Answer 14

Makes up standard solutions

Rinse with deionised water

Question 15

Pipette

Answer 15

Delivers an accurate volume called an aliquot

Rinse with solution it is to deliver

Question 16

Conical flask

Answer 16

Holds aliquot during titration

Rinse with deionised water

Question 17

What happens if burette is not rinsed with solution it is to deliver?

Answer 17

The water would dilute the added HCl and the volume of titre would be too high. Calculated concentration of HCl is too low

Question 18

What happens if conical flask is rinsed with na2co3 solution

Answer 18

Extra Na2Co3 is not present in addition to the known amount of sodium carbonate that came from the pipette.

Extra amount of na2co3 means extra volume of titres. Titres too big so concentration of HCl calculated is too low

Question 19

Why is volumetric flask rinsed with water

Answer 19

Amount of Na2Co3 is the same as water will be added regardless to the calibration line to produce the standard solution

Question 20

What happens if pipette is not rinsed with sodium carbonate solution

Answer 20

This dilutes the Na2Co3 so the amount of sodium carbonate in the aliquot is less than it should be. Titres are too small.
Calculated concentration of HCl too high

Question 21

What happens if endpoint was overshot

Answer 21

Too much HCl added from burette. Titres too big and amount of HCl too high so,ethics

Question 22

What happens if sodium carbonate was partially hydrated

Answer 22

For a given mass of powder there will be less actual na2Co3 as some of the mass is h2o. So titres of HCl a lower volume than they should be.
Calculated concentration of HCl too high

Question 23

Why is so and so a good indicator for this titration?

Answer 23

So and so changes colour between 1-3. The ph at the equivalence point is 3. This indicator captures the equivalence point and so end point will be a good approximation of equivalence point

Question 24

What happens if an indicator who changes colour at a lower ph was used

Answer 24

Methyl orange changes colour between so and so, as such the colour change will be well before the equivalence point and the volume of the titre will be too low

Question 25

What happens if weak acid and weak base used in a titration

Answer 25

HF weak acid
NH3 weak base
Large volume of NH3 from the burette would be needed to change pH markedly. End point not sharp