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7.1 A standard solution is prepared by dissolving 10 g of sodium carbonate, Na2CO3(s), in 0.7 dm³ of water - English General - NSC Physical Sciences - Question 7 - 2024 - Paper 2

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7.1 A standard solution is prepared by dissolving 10 g of sodium carbonate, Na2CO3(s), in 0.7 dm³ of water. 7.1.1 Calculate the concentration of the solution. 7.1.... show full transcript

Worked Solution & Example Answer:7.1 A standard solution is prepared by dissolving 10 g of sodium carbonate, Na2CO3(s), in 0.7 dm³ of water - English General - NSC Physical Sciences - Question 7 - 2024 - Paper 2

Step 1

7.1.1 Calculate the concentration of the solution.

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Answer

To calculate the concentration of the sodium carbonate solution, we use the formula:

C=mM×VC = \frac{m}{M \times V}

Where:

  • mm = mass of sodium carbonate = 10 g
  • MM = molar mass of Na2CO3 = 106 g/mol
  • VV = volume of the solution = 0.7 dm³

Substituting the values, we get: C=10106×0.7=0.13 mol/dm3C = \frac{10}{106 \times 0.7} = 0.13 \text{ mol/dm}^3

Step 2

7.1.2 Will the pH of the solution be GREATER THAN or LESS THAN 7?

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Answer

The pH of the sodium carbonate solution will be GREATER THAN 7.

This is because sodium carbonate is a salt derived from a weak acid (carbonic acid) and a strong base (sodium hydroxide), leading to basic conditions in the solution.

Step 3

7.1.3 Write an equation that explains the answer to QUESTION 7.1.2.

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The dissociation of sodium carbonate in water can be represented as follows:

Na2CO3(aq)+H2O(l)2Na+(aq)+HCO3(aq)+OH(aq)Na_2CO_3(aq) + H_2O(l) \rightleftharpoons 2Na^+(aq) + HCO_3^-(aq) + OH^-(aq)

Here, the generation of hydroxide ions (OHOH^-) confirms that the solution is basic, leading to a pH greater than 7.

Step 4

7.1.4 Which ONE of the indicators (P, Q or R) is most suitable for this titration?

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Answer

The most suitable indicator for the titration of sodium carbonate with hydrochloric acid is indicator P, which has a pH range of 3.4 to 4.5.

This choice is appropriate because the equivalence point of the titration will be below pH 7 due to the weak base (sodium carbonate) reacting with a strong acid (hydrochloric acid).

Step 5

7.2.1 What is meant by a dilute acid?

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Answer

A dilute acid is an acid that has a relatively low concentration of hydrogen ions (H+H^+) compared to a concentrated acid. This means that the acid is mixed with a significant amount of water, resulting in a solution where the pH is generally higher than that of a concentrated acid.

Step 6

7.2.2 Calculate the pH of the final solution.

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First, we calculate the moles of [H+][H^+] ions:

  • Since H2SO4 is a diprotic acid, it produces 2 moles of H+H^+ for every mole:
  • Moles of H+H^+ from H2SO4: 0.01extmol×2=0.02extmol0.01 ext{ mol} \times 2 = 0.02 ext{ mol}.
  • Moles of OHOH^- from KOH: 0.024 mol.

Since OHOH^- exceeds H+H^+, we calculate the excess moles:

  • Excess OHOH^-: 0.0240.02=0.004extmol0.024 - 0.02 = 0.004 ext{ mol} in 0.2 dm³.

Concentration of OHOH^-: [OH]=0.0040.2=0.02extmol/dm3[OH^-] = \frac{0.004}{0.2} = 0.02 ext{ mol/dm}^3

Using the relation: pOH=log[OH]=log(0.02)1.7pOH = -\log[OH^-] = -\log(0.02) \approx 1.7

Then, we find the pH using: pH+pOH=14pH=141.7=12.3pH + pOH = 14 \Rightarrow pH = 14 - 1.7 = 12.3

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