8 This question is about acids and alkalis.
8-1 Explain why the pH of an acid depends on:
• The strength of the acid
• The concentration of the acid.
[4 marks]
ANSWER
Rule of Thumb
pH depends on H+ ion concentration i.e. the higher the concentration of H+ ions, the lower the pH value
The strength of the acid
The stronger an acid, the greater the ionization / dissociation (in aqueous solution) so the stronger the acid and hence lower the pH value.
The concentration of the acid
The higher the concentration of an acid, the more acid / solute in the same volume of solution so higher the concentration of the acid and hence lower the pH.
8-2 A student titrated 25.00 cm3 of hydrochloric acid with 0.100 mol/dm3 barium hydroxide solution.
Table 2 shows the results
Table 2
The student calculated the volume of barium hydroxide solution to be used in the titration calculation as 23.50 cm3.
Explain why the student used a volume of 23.50 cm3 of barium hydroxide solution in the titration calculation.
[2 marks]
ANSWER
The mean of titration numbers 2 to 5 values is calculated because 23.90 (cm3) is an anomalous result.
8-3 25.00 cm3 of the hydrochloric acid reacted with 23.50 cm3 of the 0.100 mol/dm3 barium hydroxide solution.
The equation for the reaction is:
2 HCl(aq) + Ba(OH)2(aq) → BaCl2(aq) + 2 H2O(l)
Calculate the concentration of the hydrochloric acid in mol/dm3.
[4 marks]
ANSWER
Step1: Calculating Moles of barium hydroxide solution,
Step 2: Now seeing the balanced equation, we need to identify molar ratio
HCl: Ba(OH)2 = 2:1
Therefore, moles of HCl is
Another student titrated sulfuric acid with barium hydroxide solution. The equation for the reaction is:
H2SO4 (aq) + Ba (OH)2(aq) → BaSO4(s) + 2 H2O(l)
The student measured the electrical conductivity of the mixture during the titration. The better a conductor, the higher the electrical conductivity value.
Figure 10 shows the results.
Figure 10
8-4 Explain why the electrical conductivity of the mixture was zero when the sulfuric acid had just been neutralized.
Use the equation for the reaction. Refer to ions in your answer.
Marks]
ANSWER
There are no ions that are free to move because barium sulfate is insoluble solid and hydrogen ions have reacted with hydroxide ions to produce water.
8-5 The student then added a further 10 cm3 of barium hydroxide solution. The electrical conductivity of the mixture increased.
Give one reason why.
[1 mark]
ANSWER
The mixture now contains barium ions and hydroxide ions which are free to move.
