grade 12 acid and base questions


QUESTION 7 (Start on a new page.)


7.1 Nitric acid (HNO3), an important acid used in industry, is a strong acid.
7.1.1 Give a reason why nitric acid is classified as a strong acid. (1)
7.1.2 Write down the NAME or FORMULA of the conjugate base of nitric acid. (1)
7.1.3 Calculate the pH of a 0,3 mol∙dm-3 nitric acid solution. (3)
7.2 A laboratory technician wants to determine the percentage purity of magnesium oxide. He dissolves a 4,5 g sample of the magnesium oxide in 100 cm3 hydrochloric acid of concentration 2 mol∙dm-3
.
7.2.1 Calculate the number of moles of hydrochloric acid added to the magnesium oxide. (3)
He then uses the apparatus below to titrate the EXCESS hydrochloric acid in the above solution against a sodium hydroxide solution

7.2.2 Write down the name of apparatus Q in the above diagram. (1)
7.2.3 The following indicators are available for the titration:

Which ONE of the above indicators (A, B or C) is most suitable to indicate the exact endpoint in this titration? Give a reason for the answer. (3)

7.2.4 During the titration, the technician uses distilled water to wash any sodium hydroxide spilled against the sides of the Erlenmeyer flask into the solution.
Give a reason why the addition of distilled water to the Erlenmeyer flask will not influence the results. (1)
7.2.5 At the endpoint of the titration he finds that 21 cm3 of a 0,2 mol dm-3 sodium hydroxide solution has neutralised the EXCESS hydrochloric acid. Calculate the number of moles of hydrochloric acid in excess. (3)
7.2.6 The balanced equation for the reaction between hydrochloric acid and magnesium oxide is:
MgO(s) + 2HCℓ(aq) → MgCℓ2(aq) + 2H2O(ℓ)
Calculate the percentage purity of the magnesium oxide. Assume that only the magnesium oxide in the 4,5 g sample reacted with the acid. (5)
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