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
1. To calculate the pH of the NaOH solution, we can use the equation:
pOH = -log[OH-]
The concentration of the NaOH solution is given as 0.15 mol/dm3. Since sodium hydroxide is a strong base, it dissociates completely in water to form OH- ions. Therefore, the concentration of OH- ions is also 0.15 mol/dm3.
Now, we can calculate pOH:
pOH = -log(0.15) = 0.823
To convert pOH to pH, we can use the equation:
pH = 14 - pOH = 14 - 0.823 = 13.177
Therefore, the pH of the NaOH solution is approximately 13.177.
2. The titration is repeated more than once to obtain more accurate and reliable results. By repeating the titration multiple times, any errors or inconsistencies in the measurements can be minimized. Taking an average of the results also helps to reduce the impact of any outliers or random errors.
3. The balanced reaction for the titration is:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
This reaction shows that sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to produce sodium chloride (NaCl) and water (H2O).
4. To calculate the concentration of the HCl solution, we can first use the balanced equation to determine the mole ratio between NaOH and HCl. From the balanced equation, we can see that the ratio is 1:1, meaning one mole of NaOH reacts with one mole of HCl.
Given that the volume of the NaOH solution used in the titration is 25 cm3 and its concentration is 0.15 mol/dm3, we can calculate the amount of NaOH used:
Amount of NaOH = concentration x volume = 0.15 mol/dm3 x 0.025 dm3 = 0.00375 mol
Since the mole ratio is 1:1, the amount of HCl used in the titration is also 0.00375 mol.
The average volume of HCl used in the titration is given as 30 cm3. Therefore, the concentration of the HCl solution can be calculated:
Concentration of HCl = amount of HCl / volume of HCl solution = 0.00375 mol / 0.030 dm3 = 0.125 mol/dm3
Therefore, the concentration of the HCl solution is 0.125 mol/dm3.