Learners perform a titration to standardise a dilute hydrochloric acid solution.
They use 25 cm3
of a standard 0,15 mol∙dm–3 sodium hydroxide (NaOH) solution.
The titration is repeated three times, the average HCl volume readings are calculated and found
to be 30 cm3
1. Calculate the pH of the NaOH solution.
2. Give a reason why the titration is repeated more than once
3. The balanced reaction for the above titration is:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2
O(l)
Calculate the concentration of the HCl.
4. The following indicators are available:
Indicator pH Range
Ind1 3,1 – 4,4
Ind2 6,0 – 7,6
Ind3 8,3 – 10,0
5. Which indicator will be the most appropriate to use and why?
6. Give the name of that indicator
1. To calculate the pH of the NaOH solution, we need to use the equation for the reaction between NaOH and HCl:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
The balanced equation shows that one mole of NaOH reacts with one mole of HCl. Since the concentration of the NaOH solution is given as 0.15 mol/dm^3 and the volume used is 25 cm^3, we can calculate the number of moles of NaOH used:
moles of NaOH = concentration × volume
moles of NaOH = 0.15 mol/dm^3 × 0.025 dm^3
moles of NaOH = 0.00375 mol
Since the reaction is 1:1, the moles of HCl used in the titration is also 0.00375 mol. The average volume of HCl used is 30 cm^3, so the concentration of the HCl solution can be calculated as:
concentration of HCl = moles/volume
concentration of HCl = 0.00375 mol / 0.03 dm^3
concentration of HCl = 0.125 mol/dm^3
2. The titration is repeated more than once to ensure accuracy and reliability of the results. By repeating the titration multiple times, any errors or discrepancies can be identified and the average value can be calculated to obtain a more precise concentration of the HCl solution.
3. The balanced reaction for the above titration is:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
4. To determine the most appropriate indicator to use, we need to consider the pH range of each indicator and choose one that is closest to the expected pH change during the titration. In this case, since we are titrating a strong acid (HCl) against a strong base (NaOH), the expected pH at the equivalence point will be close to 7. Therefore, the indicator with a pH range closest to 7 is the most appropriate.
5. Indicator Ind2 with a pH range of 6.0-7.6 would be the most appropriate to use as it covers the expected pH range of the equivalence point.
6. The name of the indicator with the pH range of 6.0-7.6 is not given in the question.