7.1 Define a base in terms of the Arrhenius theory. 7.2 Explain how a weak base differs from a strong base. 7.3 Write down the balanced equation for the hydrolysis of NaHCO3. 7.4 A learner wishes to identify element X in the hydrogen carbonate, XHCO3. To do this she dissolves 0.4 g of XHCO3 in 100 cm3 of water. She then titrates all of this solution with a 0.2 mol dm-3 hydrochloric acid (HCl) solution. Methyl orange is used as the indicator during the titration. 7.4.1 Calculate the pH of the hydrochloric acid solution. 7.4.2 Give a reason why methyl orange is a suitable indicator in this titration. 7.4.3 Identify element X by means of a calculation.

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7.1 Define a base in terms of the Arrhenius theory - English General - NSC Physical Sciences - Question 7 - 2019 - Paper 2

Question 7

7.1 Define a base in terms of the Arrhenius theory. 7.2 Explain how a weak base differs from a strong base. 7.3 Write down the balanced equation for the hydrolysis... show full transcript

Worked Solution & Example Answer:7.1 Define a base in terms of the Arrhenius theory - English General - NSC Physical Sciences - Question 7 - 2019 - Paper 2

Step 1

Define a base in terms of the Arrhenius theory.

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Answer

In the Arrhenius theory, a base is defined as a substance that increases the concentration of hydroxide ions (OH⁻) when dissolved in water or aqueous solution.

Step 2

Explain how a weak base differs from a strong base.

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Answer

A strong base completely dissociates in solution, resulting in a full release of hydroxide ions (OH⁻). In contrast, a weak base only partially dissociates, leading to an incomplete formation of hydroxide ions.

Step 3

Write down the balanced equation for the hydrolysis of NaHCO3.

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Answer

The balanced equation for the hydrolysis of sodium bicarbonate (NaHCO3) is:

ightleftharpoons ext{H}_2 ext{CO}_3(aq) + ext{NaOH}(aq) $$

Step 4

Calculate the pH of the hydrochloric acid solution.

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Answer

To calculate the pH of the hydrochloric acid (HCl) solution, we use the formula:

$ext{pH} = - ext{log}[ ext{H}_3 ext{O}^+]$

Given that the concentration of HCl is 0.2 mol dm⁻³:

$[ ext{H}_3 ext{O}^+] = 0.2 ext{ mol dm}^{-3}$

Thus,

$ext{pH} = - ext{log}(0.2) \\ ext{pH} ext{ is approximately } 0.70 ext{ (0.699 rounded)}$

Step 5

Give a reason why methyl orange is a suitable indicator in this titration.

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Answer

Methyl orange is suitable because it has a transition range that aligns with the pH of the solution at the endpoint of the titration. It changes color at pH values around 3.1 to 4.4, which is within the acidic range necessary for this titration between a strong acid (HCl) and a weak base (XHCO3).

Step 6

Identify element X by means of a calculation.

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Answer

To identify element X, we first find the number of moles of HCl used:

Volume of HCl = 20 cm³ = 0.020 dm³
Given concentration of HCl = 0.2 mol dm⁻³:

$n( ext{HCl}) = c imes V = 0.2 ext{ mol dm}^{-3} imes 0.020 ext{ dm}^3 = 0.004 ext{ mol}$

Since the balanced reaction is:

ightarrow ext{X} + ext{CO}_2 + ext{H}_2 ext{O} $$ It shows a ratio of 1:1, thus the moles of XHCO3 is also 0.004 mol. The mass of XHCO3 used is 0.4 g: Molar mass calculation: $$ M( ext{XHCO}_3) = rac{0.4 ext{ g}}{0.004 ext{ mol}} = 100 ext{ g mol}^{-1} $$ From the molar mass of XHCO3, we subtract the known molar masses: - Molar mass of H = 1 g/mol - Molar mass of C = 12 g/mol - Molar mass of O3 = 3 × 16 = 48 g/mol So, $$ M(X) = M(XHCO_3) - (M(H) + M(C) + M(O_3)) \ = 100 - (1 + 12 + 48) = 39 ext{ g mol}^{-1} $$ Since potassium (K) has a molar mass of approximately 39 g/mol, element X is identified as potassium.

7.1 Define a base in terms of the Arrhenius theory - English General - NSC Physical Sciences - Question 7 - 2019 - Paper 2

Worked Solution & Example Answer:7.1 Define a base in terms of the Arrhenius theory - English General - NSC Physical Sciences - Question 7 - 2019 - Paper 2

Define a base in terms of the Arrhenius theory.

Explain how a weak base differs from a strong base.

Write down the balanced equation for the hydrolysis of NaHCO3.

Calculate the pH of the hydrochloric acid solution.

Give a reason why methyl orange is a suitable indicator in this titration.

Identify element X by means of a calculation.

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