Volumetric And Gravimetric Analysis (incl Errors)

Question 1

What is Volumetric analysis?

Answer 1

VA refers to a type of QA which depends on the accurate measurements of the volumes of 2 solutions which react tgt completely

Question 2

How is Volumetric analysis done?

Answer 2

Through a titration process in which the titrant is titrated against a fixed volume of analyte until the equivalence point is reached (ie, where stoiochiometric amts of the reactants have reacted)

Question 3

[AB titration] Define the equivalence point in AB context

Answer 3

The equivalence point in an acid base titration corresponds to the point when stoichiometric amts of the acid and base have reacted. Generally involves a rapid PH change over this point

Question 4

Define what’s a PH indicator and how it works to show the eq point of a reaction

Answer 4

An indicator is a solution which undergoes a distinct colour change. When the indicator first changes colour permanently, it will mark the eq point of a reaction

Question 5

PH transition range and colours of methyl orange?

Answer 5

3.2-4.4, red to orange to yellow

Question 6

PH transition range and colours of screened methyl orange and methyl red?

Answer 6

1 . Screened methyl orange
3-5, magenta to grey to green

2 . Methyl red
4-6, Red to O to Y

Question 7

PH transition range and colours of thymolphthalein and thymol blue

Answer 7

1. 9.4-10.6, colourless to pale blue to blue
2. Used for double indicator titrations
   1.2-2.8 and 8.0-9.6
   Red to yellow to blue (primary colours!)

Question 8

How do you determine the suitability of a PH Indicator? [expln]

Answer 8

A PH indicator is deemed appropriate for the reaction when its PH transition range lies within the range of rapid ph change over the eq point. A sharp end point is produced when a distinct colour change occurs when one drop of the titrant is added.

Question 9

[redox titration] Explain why H2SO4 and not other acids (HCl, HNO3) is used to acidify potassium manganate

Answer 9

HCL reacts with MnO4- to produce Cl2
HNO3 is also a strong OA which might interfere with the oxidising action of MnO4- ions

Question 10

[redox titration] Give the equation for the reduction of MnO4- ions

Answer 10

MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O

Question 11

[VA planning] What are some precautions when dealing with KMnO4 as the titrant?

Answer 11

Conical flask should be continuously swirled (like usual) but also to prevent the formation of brown MnO2 which will make it difficult to detect the end point.

Eq point is taken when permanent pale pink colour first appears by one excess drop of KMnO4

Take reading from top of meniscus instead of bottom reading, which is obscured by the dark purple

Question 12

[iodometric titration] Describe how iodometric titrations are carried out and its purpose

Answer 12

Useful for finding the concentration of the oxidising agent in the iodometric reaction (which is I2)

Substance added to KI to liberate iodine. Amt of liberated iodine is measured by titrating with sodium thiosulfate (and using mole ratio, blah blah)

Question 13

[iodometric titration] Explain why starch solution is added and why it is added only towards the end of the reaction

Answer 13

Starch is added to detect the end point more accurately as the colour change from brown to pale yellow to colourless is less distinct

Starch added only when pale yellow Col seen. Otherwise, many I2 molecules are strongly adsorbed on the starch molecules to form the blue black complex. Removal of I2 molecules from blue black complex to react with Na2S2O3 takes time and Col change occurs less readily —> affects accuracy

Question 14

State the accuracy of burette and electronic mass balance readings (ie, to be recorded to how many dp)

Answer 14

Burette: 2dp. Even for calculations of avg burette reading

Electronic mass balance: 3dp

Question 15

State the difference (describe) between presentation of mass readings in the 2 scenarios:

Students to weigh between 4.50-5.0g of FA1, to be placed into the weighing bottle

Students to find out mass of FA1 used when sample is provided in a weighing bottle

Answer 15

For first one, don’t need to account for residue. Students will add FA1 directly into the beaker/conical flask they are using and measure its weight, so technically all the FA1 will be used

2nd one need to reweigh “mass of weighing bottle and residual FA1” bc you’re defo going to leave some behind when you transfer into your conical flask/beaker

Question 16

[preparation of standard solution from solid using volumetric flask] Describe some of the necessary precautions to take when preparing the standard solution

Answer 16

1 . Continue adding deionised water till the volume of water is about 1cm^3 below the graduation mark. From then on, use a dropper to add deionised water dropwise till the bottom of the meniscus of the water reaches the mark.

2. After reaching mark, need to insert the stopper of the volumetric flask and invert it, shaking around 10x to achieve a homogenous mixture

Question 17

What is the objective of GA? What can it be used to calculate/determine

Answer 17

Gravimetric Analysis is a method to accurately determine the masses of solid samples formed via thermal decomposition or precipitation. It enables determination of information like water of crystallisation of a salt or its relative molecular mass (others include % by mass and % purity )

Question 18

[GA] what is formed when some common solid compounds are decomposed (eg carbonate, bicarbonate, nitrate, sulfite, group 2 metal hydroxide)

Answer 18

See notes

Question 19

[GA planning] procedure for gravimetric analysis: determining mass of the solid sample formed after heating

Answer 19

Procedure:
1. Weigh an empty and dry crucible using an electronic mass balance and record its mass
2. Add a fixed mass of solid sample to the crucible. Weigh and record its final mass and the mass of solid sample added
—> no need to acct for residue
3. Heat the crucible and its contents strongly for 1 min and gently for the next 3
4. Allow it to cool to room temp
5. Reweigh crucible and its contents using an electronic mass balance and record its mass
6. Repeat steps 3-5 till 3 constant mass readings have been obtained. Record the final mass of the crucible + contents

Question 20

[error calculations] Define uncertainty (in descriptive and mathematical terms)

Answer 20

Uncertainty refers to the range of values on both sides of a measurement, in which the “true” value of the measurement is expected to lie

Question 21

Show the formula for percentage uncertainty

Answer 21

Percentage uncertainty = (uncertainty of instrument x number of readings taken) / measured value

Question 22

Is uncertainty of an instrument the same as smallest division on the instrument? What is the uncertainty for pipette and stopwatch?

Answer 22

In most cases, no. Uncertainty of instrument is usually half SD, except for stopwatch and calibrated instruments (eg, pipette and volumetric flask). For stopwatch, uncertainty = SD, ie +/- 0.01s

10.0cm3 pipette: +/- 0.02cm3
25.0cm3 pipette: +/- 0.03 cm3

Question 23

What is the smallest division on a burette? What is its uncertainty

Answer 23

Uncertainty is +/- 0.05cm3
Smallest division: +/- 0.1cm3

Note: both uncertainty and SD have units ok

Question 24

[multiplying and dividing measurements] How do you find uncertainty of density?

Answer 24

(When mass and volume are the measured quantities)

Find individual % uncertainty of both quantities (ie number of readings taken x uncertainty of instrument / measured value—> mass or volume)

Add both tgt to give total % uncertainty.

Overall uncertainty = % uncertainty x density (because the “measured values” go through the same division/multiplication of the quantities)

Question 25

Record pipette readings to how many dp?

Answer 25

1 dp

Question 26

How to calculate uncertainty for direct weighing?

Answer 26

Direct weighing involves:
1. Put empty bottle on balance and TARE (this is also considered a reading)
2. Add mass of solid to bottle and weigh

Total uncertainty = 2 (half SD of balance) = 2(+/- 0.0005) g

Question 27

How do you calculate percentage uncertainty for the mixing of volumes?

Answer 27

Addition of uncertainties (numerator) directly, divide by TOTAL volume

It’s NOT the sum of the individual percentage uncertainties

Question 28

[adding/subtracting readings] How to find uncertainty and percentage uncertainty?

Answer 28

For BOTH addition and subtraction, (just like variance), uncertainty = the ADDITION of both uncertainties.

For percentage uncertainty, either divide uncertainty by (M1-M2) or (M1+ M2)

Question 29

[BACK titration] Explain when this method is used and how the initial amount of reagent is calculated

Answer 29

Used when direct titration not possible because of: (1) lack of suitable indicator (2) one of the reagents is insoluble solid

General method:
An unknown amount of A is reacted with a known excess of B. Remaining B found w titration with standard reagent, C. Through calculations, can find original amount of A used.

Question 30

[GA] How to determine percentage purity by dehydrating a hydrated salt [ie, loss of water of crystallisation

Answer 30

1. Compare masses before and after to get water of crystallisation
2. Determine % by mass of WOC in the impure compound using this value
3. Determine regular % by mass of WOC in pure compound
4. To find percentage purity, take step 2/ step 3

Why?
% by mass in impure compound = mass of H2O / mass of impure salt
% by mass in pure = mass of H2O / mass of pure salt

% purity = mass / mass of impure salt. So you do 1/ % by mass in pure to get (mass of pure salt) as numerator, multiplied by mass of H2O / mass of impure salt. simplifying, you get mass of pure/ mass of impure

Question 31

[GA —percentage purity via water of crystallisation] What is a possible experimental error that could result in an absurdly high percentage purity? (Eg > 100%)

Answer 31

Percentage purity is essentially % by mass of WOC in impure / % by mass of WOC in pure.

% by mass of WOC in impure is determined by looking at the mass lost. A v high percentage purity can be obtained if the mass loss after heating is large.

Reason for this:
1. Some of the impurities may be hydrated too, leading to larger mass loss than usual
2. The student may have overheated the sample decomposed the anhydrous salt itself, from MgSO4 to MgO (or whatever decomposition reaction reactant and product), leading to a larger than expected mass loss.

Question 32

Describe how a standard solution is prepared (planning) from a solid reactant.

Answer 32

1. Using an electronic mass balance, weigh a weighing bottle containing __ g of solid.
2. EMPTY the contents of the weighing bottle into a 100cm3 beaker. Reweigh the mass of the weighing bottle containing residual solid. Record the mass and calculate mass of solid used.
3. To the beaker, add about 50cm3 of deionised water and stir using a glass rod to dissolve the solid.
4. Transfer contents of beaker into a 100cm3 volumetric flask. Rinse the beaker with deionised water, transferring washings to the volumetric flask.
5. Add deionised water till liquid level reaches graduation mark.
6. Stopper the flask and invert it a few times to ensure a HOMOGENOUS solution.

Question 33

[VA] when using acidified KMnO4 as the oxidising agent in a titration reaction, explain why it is unnecessary to use a burette/pipette to measure the volume of acid added

Answer 33

Sulfuric acid used merely provides an acidic medium for titration to occur. Moreover, it is used in excess, hence vol of acid added will not affect the titre volume

Question 34

[QA] When heating solids which gives off water when heated, why is it important to hold the test tube slightly sloping downwards?

Answer 34

Prevents the condensed water at the mouth of the test tube from flowing back to the hot part of the test tube and cracking the glass