Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.4.3Atomic Structure and the Periodic Table

Concentration of an Unknown acid

5 min read 7 parts Lesson 1 of 10 Unit quiz

0%read

Section 1 of 7Aim and equipment

Aim
  • Aim To find the volume of sulfuric acid of unknown concentration that exactly neutralises a fixed volume of sodium hydroxide of known concentration,
    • then calculate the acid's concentration.
Apparatus
  • Apparatus 25 cm3 volumetric pipette with filler, 50 cm3 burette, conical flask, small funnel, white tile, clamp stand, methyl orange indicator.
  • 0.1 mol/dm3 sodium hydroxide solution and sulfuric acid of unknown concentration.
Variables
  • Variables Independent variable: volume of acid added.
    • Dependent variable: the colour of the indicator, which shows the end-point.
  • Control variablesvolume and concentration of alkali, type and amount of indicator.

Section 2 of 7Method

1
Use the pipette and filler to transfer exactly 25.0 cm3 of sodium hydroxide into the conical flask, and add a few drops of methyl orange.
2
Clamp the burette upright. Use a funnel to fill it with the acid, then remove the funnel and run a little acid through the tap to remove air bubbles. Record the starting reading to the nearest 0.05 cm3, with your eye level with the meniscus.
3
Stand the flask on a white tile under the burette tip. Add acid in 1 to 3 cm3 portions, swirling continuously, until the indicator changes from yellow to orange or red. This is the rough titration.
4
Repeat with fresh alkali, adding acid quickly to about 3 cm3 before the rough value, then drop by drop until one drop makes the colour change. Record the final reading.
5
Repeat until you have two concordant results, meaning titres within 0.10 cm3 of each other.

Section 3 of 7Recording and processing results

RunFinal reading (cm3)Start reading (cm3)Titre (cm3)
RoughRecordedRecordedFinal minus start; not used in the mean
Titre 1RecordedRecordedFinal minus start
Titre 2RecordedRecordedFinal minus start
Mean titre--Add the concordant titres and divide by the number of them

Section 4 of 7Safety and good technique

Reliable titration
Safety
  • Safety Wear eye protection.
    • Sodium hydroxide is corrosive and sulfuric acid is an irritant;
      • wipe up spills at once.
Funnel removed
  • Funnel removed A funnel left in the burette can drip and change the reading.
White tile and swirling
  • White tile and swirling The tile makes the first permanent colour change easy to see,
    • and swirling mixes the reactants so the end-point is not overshot.
Rough titration
  • Rough titration It shows roughly where the end-point is so later runs can be done slowly near it.

Section 5 of 7Using the result

1Moles of alkali
  • Moles of alkali Moles NaOH = 0.1 × 0.025 = 0.0025 mol.
2Use the equation
  • Use the equation 2NaOH + H2SO4 → Na2SO4 + 2H2O, so moles of acid are half those of alkali.
3Concentration
  • Concentration Divide the moles of acid by the mean titre in dm3 to get the concentration in mol/dm3.

Section 6 of 7Key terms

TermMeaning
TitrationA method using a burette to find the volume of one solution that reacts exactly with a measured volume of another.
TitreThe volume of solution added from the burette to reach the end-point.
Concordant resultsRepeat titres that agree to within 0.10 cm3.
End-pointThe point where the indicator just changes colour.
MeniscusThe curved surface of a liquid; read the bottom of it at eye level.

Section 7 of 7Exam technique

1
Including the rough titre in the mean. Only concordant titres are averaged.
2
Leaving the funnel in the burette, which can drip into it and give a false starting reading.
3
Reading the burette from above or below instead of at eye level with the meniscus.
4
Forgetting that sulfuric acid reacts with sodium hydroxide in a 1:2 ratio, not 1:1.
\n