RP1 - Making up a standard solution & carrying out an acid-base titration

Question 1

how to make a standard solution

Answer 1

Alternatively the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker.
Weigh the sample bottle containing the required mass of solid on a 2 dp balance
* Transfer to beaker and reweigh sample bottle
* Record the difference in mass
* Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
*Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved.
* *
*
Pour solution into a 250cm3 graduated flask via a funnel. Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.

Make up to the mark with distilled water using a dropping pipette for last few drops.
* Invert flask several times to ensure uniform solution.
Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate it can be difficult to see the meniscus.

Question 2

xib

Answer 2

The method for carrying out the titration
*rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
*pipette 25 cm3 of alkali into conical flask
*touch surface of alkali with pipette ( to ensure correct amount is added)
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point *phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH is used]
*methyl orange [yellow (alkali) to red (acid): end point orange]
[use if HCl is used]
*use a white tile underneath the flask to help observe the colour
change
*add acid to alkali whilst swirling the mixture and add acid dropwise at end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other

Answer 3

Safety precautions
Acids and alkalis are corrosive
(at low concentrations acids are irritants)
Wear eye protection and gloves
If spilled immediately wash affected parts after spillage
If substance is unknown treat it as potentially toxic and wear gloves

Answer 4

If the jet space is not filled properly prior to commencing the titration it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading.

Answer 5

A conical flask is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling the contents.

Answer 6

Indicators are generally weak acids so only add a few drops of them. If too much is added it will affect the titration result.

Answer 7

Distilled water can be added to the conical flask during a titration to wash the sides of the flask so that all the acid on the side is washed into the reaction mixture to react with the alkali.
It does not affect the titration reading as water does not react with the reagents or change the number of moles of acid added.

Answer 8

Working out average titre results
Only make an average of the concordant titre results

Answer 9

lf 2 or 3 values are within 0.10cm3 and therefore concordant or close then we can say results are accurate and repeatable and the titration technique is good/ consistent

Answer 10

Recording results
*Results should be clearly recorded in a table
*Result should be recorded in full (i.e. both initial and final readings) *Record titre volumes to 2dp (0.05 cm3)

Answer 11

Titrating mixtures
If titrating a mixture to work out the concentration of an active ingredient it is necessary to consider if the mixture contains other substances that have acid base properties.
If they don’t have acid base properties we can titrate with confidence.

Answer 12

Testing batches
In quality control it will be necessary to do titrations/testing on several samples as the amount/concentration of the chemical being tested may vary between samples.

Answer 13

Common Titration Equations
CH3CO2H + NaOHCH3CO2-Na+ + H2O
H2SO4 + 2NaOH  Na2SO4 +2H2O HCl + NaOH  NaCl +H2O
NaHCO3 + HClNaCl + CO2 + H2O Na2CO3 + 2HCl 2NaCl + CO2 + H2O

Answer 14

Safely dealing with excess acid
Sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3) are good for neutralising excess acid in the stomach or acid spills because they are not corrosive and will not cause a hazard if used in excess. They also have no toxicity if used for indigestion remedies but the CO2 produced can cause wind.
Magnesium hydroxide is also suitable for dealing with excess stomach acid as it has low solubility in water and is only weakly alkaline so not corrosive or dangerous to drink (unlike the strong alkali sodium hydroxide). It will also not produce any carbon dioxide gas.

Question 15

pg13 chem revise