2.2 Flashcards
2.2 Determination of the molar mass of an acid
• You will prepare a solution of a hydrated acid, H3X•nH2O.
• You will then carry out a titration of this solution with 0.100 mol dm–3 sodium hydroxide, NaOH.
From your results, you will determine the molar mass of the acid.
• You will then calculate the value of n in the formula H3X•nH2O using some additional data.
hazard of A solid acid, H3X•nH2O
Irritant
hazard of Aqueous sodium hydroxide, NaOH, of concentration 0.100 mol dm–3
Irritant
hazard of Phenolphthalein as indicator
Highly flammable
Equipment
Equipment • Safety spectacles • Burette • White tile • Pipette (25.0 cm3) and filler • Clamp stand, with boss and clamp (for supporting the burette) • Filter funnel • Measuring cylinder (100 cm3) • Glass rod • Dropping pipette • Volumetric flask and stopper (250 cm3) • Wash bottle containing distilled (or deionised) water (about 300 cm3 will be required) • Two conical flasks (250 cm3) • Glass beaker (250 cm3 and 100 cm3)
Each group will also need access to the following:
• Top-pan balance weighing to 0.01 g
• Dropping bottle containing phenolphthalein indicator
Procedure
- Weigh the bottle provided, containing H3X•nH2O.
- Tip the solid into a 250 cm3 beaker and re-weigh the empty bottle.
- Dissolve the solid carefully in about 100 cm3 of distilled (or de-ionised) water.
- Transfer this solution into a 250 cm3 volumetric flask.
- Make the solution up to 250 cm3 using distilled water.
- Invert the volumetric flask several times before use to mix the solution thoroughly.
- Using a pipette and filler, transfer 25.0 cm3 of the aqueous sodium hydroxide into a conical flask.
- Add 3–4 drops of phenolphthalein indicator.
- Fill the burette with some of the solution that you prepared in the volumetric flask.
- Record all burette readings to the nearest 0.05 cm3.
- Carry out a trial titration.
- The colour change at the end-point is from pink to colourless.
- Now carry out the titration accurately and obtain two concordant values for the titre
Analysis
The equation below represents the reaction that you carried out.
3NaOH(aq) + H3X(aq) Na3X(aq) + 3H2O(l)
- Calculate the amount, in moles, of NaOH used in your mean titration.
- Using the equation above, calculate the amount, in moles, of H3X in your mean titre.
- Calculate the amount, in moles, of H3X present in the 250 cm3 solution that you prepared.
- Calculate the molar mass, in g mol–1, of H3X•nH2O.
- In another experiment a teacher obtained a molar mass for H3X•nH2O of 210 g mol–1.
Use this information and the structure of the anhydrous acid H3X below to calculate the value of n in the formula H3X•nH2O.
- The balance has a maximum error of ±0.005 g in each reading. Calculate the percentage error in your mass of sodium hydrogencarbonate.
- Calculation assuming the mass has been determined from two mass readings:
% error = × 100 = 0.48%
- Explain why hydrated sodium carbonate and sodium hydroxide are not appropriate bases for making up a standard solution.
- The substance used to make a standard solution must have an accurately known molar mass, so we know exactly how many moles we have dissolved in a given volume.
Hydrated sodium carbonate loses water of crystallisation to the atmosphere (it is efflorescent), so its molar mass is variable.
Sodium hydroxide absorbs moisture from the air (it is deliquescent), so it also has a variable molar mass.
- The burette has a maximum uncertainty of ±0.05 cm3 in each reading. Calculate the percentage error in your first accurate titre.
- This will depend on student results but using specimen results
- The volumetric flask has a maximum error of ±0.3 cm3 and the pipette has a maximum uncertainty of ±0.04 cm3. Which of these gives the larger percentage error?
- % error for volumetric flask = 0.3/250 × 100 = 0.12%
% error for pipette = 0.04/25 × 100 = 0.16%
Therefore, the pipette has the greater % uncertainty.
hazard H3X•nH2O
Irritant
hazard NaOH(aq)
Irritant
hazard Phenolphthalein
indicator solution
Highly flammable
