Core Practicals Flashcards
What is a standard solution
A solution with known concentration
How do you make a standard solution
- Measure mass of required solid with a balance
- Transfer this to a volumetric flask and rinse the remaining bottle content (with distilled water) into the flask so no solid is lost
- Add a volume of distilled water to dissolve the solid and swirl
- Then add more distilled water up to the line on the neck of the volumetric flask and invert multiple times to mix
What is the standard solution for finding concentration of sodium hydroxide
- Dilute sulfamic acid
- Dissolve 2.5g of solid sulfamic acid in 100cm3 of distilled water
- Transfer up to 250cm3 volumetric flask and fill up ti line with distilled water
What is the weighing by difference technique?
- It is a method to weigh materials accurately
- Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred
What equipment is used to carry out the titration for sodium hydroxide
- A pipettes and pipette filler are used to accurately measure out the volume of NaOH before transferring it to a conical flask
- A Burette is used to add small volumes of sulfamic acid solution to the NaOH until the reaction has reached completion
How do you carry out a titration?
- Once the pipette has been used to place NaOH into the conical flask, fill the burette with the acid solution. Record initial volume
- Ass a few drops of methyl orange to the conical flask
- Open burette tap and allow the sulfamic acid to flow into the conical flask, swirling it
- Close the burette tap once the expects colour ch she occurs. Use a white tile to easily identify this
- Record final burette volume
- Repeat until you get concordant results then calculate a mean titre
Why are acid-base indicators used
To detect when a reaction reaches completion, usually by the presence of a colour change
What are concordant results?
Titres that are within 0.1cm3 of each other
How would you analyse results from sodium hydroxide titration
- Calculate the concentration of the sulfamic solution (if not already known)
- Calculate the mean titre using concordant results
- Calculate the concentration of the burette solution
Bunsen burner : risk and control
Risk - burns
Control - Keep away from flammable chemicals and away from the edge of the desk
Chemicals : risk and control
Risk - May be an irritant or corrosive. May be toxic. May be flammable
Control - Handle with car while wearing gloves. Wear eye protection. Keep away from the edge of the desk and from and open flame and do not ingest
How do you make a diluted solution of hydrochloric acid?
- Add 25cm3 of the hydrochloric acid solution into the volumetric flask using a pipette
- Make the solution up to the line by adding distilled water
How do you analyse the results from the NaOH HCl titration
- Calculate the mean titre using concordant results
- Calculate the number of moles of NaOH in the mean titre n=cv
- Calculate the number of moles of HCl that were present in the 250cm3 diluted solution
- Using this to calculate the concentration of the original solution of HCl
Glassware (beakers, test tubes) : risk and control
Risk - When broken could cut you
Control - Handle with care and keep away from the edge of the desk
Why are acid-base indicators used?
To detect when a reaction reaches its equivalence point. The indicator should be chosen so that it’s end point (point of colour change) matched the equivalence point of the reaction
Why does a pH probe need to be calibrated?
So that for each pH reading, the pH value is accurate
How do you calibrate a pH probe?
Submerge the pH probe in buffer solution of three different pHs including pH 7 and usually pHs around 4 and 10. Each time pressing calibrate
What is accuracy
The more accurate the data, the closer it is to the actual value
How do you carry out a titration to calculate pH?
- Into a conical flask, add 25cm3 of 0.1 moldm-3 ethanoic acid solution with a few drops of phenolphthalein
- The sodium hydroxide solution goes into the burette
- Titrate the solutions together until it just turns pale pink
- Add another 25cm3 of 0.1 moldm-3 ethanoic acid solution into the conical flask using a pipette
- Record the pH of the resulting solution
How do you calculate Ka from titration to find pH
- In the solution only half the acid has been titration therefore : [HA] = [A-]
- You can then cancel [A-] and [HA] in the Ka equation as they equal the same thing so Ka = [H+]
- Convert the resulting solutions pH to [H+] to give a value for the acid disassociation constant, Ka. [H+]=10^-pH
What is a redox titration?
A titration of a reducing agent by and oxidising agent or the other way around
How would you find the amount of iron in an iron tablet experimentally?
- Titrate potassium manganate (VII) solution against acidified (H2SO4) crushed iron tablets
- Calculate mean titre using concordant results
- Calculate number of moles of MnO4- in the mean titre
- Using stoichiometry calculate the number of moles of Fe2+ reacted and in the original solution
- Using this calculate the mass of Fe in solution and mass per tablet
How would you reduce uncertainty in a redox titration
- Use a balance with greater resolution
- Use pipettes and burette with greater resolution
- Use greater masses and volumes so percentage uncertainty is reduced
What is the weighting by difference technique (used to measure mass of the iron tablets) ?
- It is a method to weigh materials accurately
- Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred
