Risk Assessment
Calcium Hydroxide (Ca(OH)2)
- Hazards – Mildly Corrosive, may cause skin and eye damage if it isn’t washed off quickly
- Safe Handling – Wear safety goggles
- Emergency – Eye contact: Immediately wash with lots of water. If irritation continues seek medical help. Skin Contact: Wash off with soapy water. If Swallowed: If the person is conscious wash out their mouth with water. Call for medical help immediately.
Hydrochloric Acid (HCl)
- Hazards – Contact with eyes or skin can cause serious permanent damage. Concentrated solutions are very corrosive but mild dilutions like the one used present a smaller risk. Concentrated Hydrochloric acid releases dangerous vapour.
- Safe Handling – Wear safety goggles. Avoid contact with skin.
- Emergency – Eye Contact: Wash immediately with water for at least 10 minutes. Call for medical help. Skin Contact: Wash off with plenty of water and remove any clothing that has been in contact with the acid. If skin reddens or is damaged seek medical aid. If Swallowed: Drink lots of water. Seek immediate medical aid.
Methyl Orange ((CH3)2NC6H4:NC6H4SO3Na)
- Hazards – Is toxic if swallowed or inhaled. Avoid skin contact.
- Safe Handling – Wear Safety glasses. Wash off if skin contact occurs
- Emergency – Eye Contact: Wash with water. If irritation doesn’t subside seek medical aid. Skin Contact: Wash with plenty of water. If Swallowed: Drink plenty of water and call for medical help.
References
M2.4 Experiment Sheet (Salter’s Advanced Chemistry)
Lab Mouse 2 (BNFL)
Risk Assessment Sheets (School Science Review/CLIAPS)
Avogadro Website
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HSci Chemical Safety Database (Used for risk assessment)
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Results
Analysis
Dilution of acid
(Initial Volume/Total Volume) x Initial Concn = New Concn
(25 cm3 / 250 cm3) x 0.3 mol dm-3 = 0.03 mol dm-3
Titration Average
(A+B)/2 = C
(26.30 cm3 + 26.20 cm3)/2 = 26.25 cm3
Amount of Acid to Neutralise Ca(OH)2
(Volume/1000) x Concentration of Dilute Acid = Amount of Moles of Acid
(26.25 cm3 / 1000) x 0.03 mol dm-3 = 0.0007875
= 7.88 x 10-4 mol
Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) + 2H2O(l)
Ratio (Ca(OH)2 : 2HCl) = 1:2
Amount in Moles of Ca(OH)2 In solution
(Amount of Acid/2) = Amount of Calcium Hydroxide
(7.88 x 10-4 mol/2) = 3.94 x 10-4 mol
Concentration of Ca(OH)2 Solution
(Amount of Ca(OH)2 / Volume of Ca(OH)2) = Concentration of Ca(OH)2
(3.94 x 10-4 mol / 0.025 dm-3) = 0.01576 mol dm-3
= 0.016 mol dm-3
Evaluation
There are several limitations associated with using the above procedure.
- All the glassware used has errors.
- The end point of the indicator is not easy to resolve and requires a bit of practise and some skill with wet chemistry.
- If the glassware is not correctly washed with distilled water then the appropriate chemical contamination will occur.
- Taking readings on the glassware also requires some skill.
- The amount of indicator used (If it isn’t equal each time the end point will be shifted.)
- Mixing of the dilution must be thorough or the concentration in the dilution won’t be equal in the flask.
The most important of the points mentioned above in relation to the reliability and precision of the data collected are, the rinsing of the glassware and the mixing of the acid dilution.
The rinsing is of particular importance because if the glassware isn’t correctly rinsed contamination of the chemicals will occur. The contamination would only have a negligible effect on the dilution of the hydrochloric acid if the flask wasn’t correctly rinsed. The same would be true for the rinsing of the burette (assuming it had been washed after use). If the pipette isn’t correctly rinsed between transferring the hydrochloric acid into the volumetric flask and transferring calcium hydroxide into the conical flask, the end point of the titration could be shifted considerably caused by the hydrochloric acid remaining in the pipette neutralising some of the calcium hydroxide before the titration has begun.
Mixing also affects the end point of the titration. If the hydrochloric acid dilution isn’t mixed properly there will be a higher concentration of acid at the bottom of the volumetric flask than at the top, this will affect the end point because the lower concentration will give lower titre results than the high concentration.
There is a built in uncertainty on all of the glassware used in the titration this can be calculated as a percentage by using the glassware’s uncertainty value and the amount measured using the glassware.
(Glassware Uncertainty/Measured Amount) x 100
The uncertainty values for the glassware used in the titration are:-
-
Burette 50 cm3 Grade B +/- 0.1 cm3
-
Volumetric Pipette 25 cm3 Grade B +/- 0.06 cm3
-
Volumetric Flask 250 cm3 Grade B +/- 0.15 cm3
To calculate the total percentage error of the glassware I need to do three percentage calculations and then add all the results.
Burette
(0.1cm3 / 26.25cm3 (Titre Ave)) x 100 = 0.381% (3SF)
= 0.38%
Pipette
Multiplied by two because pipette was used twice
((0.06 cm3 / 25 cm3) x 2)) x 100 = 0.48%
= 0.48%
Volumetric Flask
(0.15 cm3 / 250 cm3) x 100 = 0.06%
= 0.06%
Total Error Percentage
0.38% + 0.48% + 0.06% = 0.92%
=0.92%
In conclusion from the above evaluation I think that the washing of the glassware has the most significant affect on the final titre result. This is because if not done correctly it can considerably shift the end point of the titre giving unreliable results; it could also make it difficult to get concordant results. Second to this would be improper mixing of the hydrochloric acid dilution I don’t think that the end point would be shifted as far but concordant results would be difficult to achieve because the concentration of the acid would be increasing as it was used up giving a smaller titre each time. The glassware error is insignificant as it is less than 1% so the final result would be reliable as long as the glassware has been used correctly.
