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a) Figure 5 shows the structure of a molecule of compound S - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2023 - Paper 1
Question 6
a) Figure 5 shows the structure of a molecule of compound S. (i) Use Figure 5 to deduce the empirical formula of compound S. (ii) The melting points of three sampl... show full transcript
Worked Solution & Example Answer:a) Figure 5 shows the structure of a molecule of compound S - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2023 - Paper 1
Step 1
Use Figure 5 to deduce the empirical formula of compound S.
Answer
To determine the empirical formula from the structure shown in Figure 5, we need to count the number of each type of atom present in the molecule.
From the figure, we can identify:
- Carbon (C): 6 atoms
- Hydrogen (H): 13 atoms
- Oxygen (O): 1 atom
The empirical formula, which represents the simplest whole-number ratio of the elements, can be derived as follows:
- Identify the smallest number of atoms. Here, the number of oxygen atoms is 1.
- Divide the number of each type of atom by the smallest number of atoms.
- C: 6/1 = 6
- H: 13/1 = 13
- O: 1/1 = 1
Thus, the empirical formula of compound S is C₆H₁₃O.
Step 2
State whether each of these samples, A, B and C, is pure or impure and justify your answers using the information in Figure 6.
Answer
To evaluate the purity of samples A, B, and C, we must analyze their melting points as shown in Figure 6.
- Sample A has a melting point range of 160-164 °C.
- Sample B has a melting point of 166 °C.
- Sample C has a melting point range of 163-165 °C.
Analysis:
- A pure substance has a specific and consistent melting point, while an impure substance has a melting point range.
- Since Sample B has a singular melting point (166 °C), it is likely pure.
- Samples A and C show ranges of melting points, suggesting that they are impure. Thus:
- Sample A: Impure, due to melting point range.
- Sample B: Pure, as it has a precise melting point.
- Sample C: Impure, due to melting point range.
Step 3
Calculate the distance the spot of compound S moves if the solvent front has moved by 2.4 cm.
Answer
To calculate the distance that the spot of compound S moves, we use the Rf value formula:
Rearranging gives:
Substituting the values:
- Rf = 0.22
- Distance moved by solvent front = 2.4 cm
Calculating:
Thus, the distance moved by the spot of compound S is approximately 0.53 cm.
Step 4
Explain this difference in boiling points in terms of the structure and bonding of sodium chloride and water and how this difference is used to choose a method to separate sodium chloride solution into pure, dry sodium chloride and pure water.
Answer
The boiling points of sodium chloride (1465 °C) and water (100 °C) differ significantly due to their distinct molecular structures and bonding.
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Structure and Bonding:
- Sodium chloride is an ionic compound formed from the electrostatic attraction between sodium ions (Na⁺) and chloride ions (Cl⁻). This strong ionic bond requires a considerable amount of energy to break, resulting in a high boiling point.
- Water, on the other hand, is a covalent molecule with weaker hydrogen bonds between its molecules. As a result, less energy is needed to separate water molecules, leading to a lower boiling point.
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Separation Method:
- The significant difference in boiling points allows for a simple separation technique: evaporation. When a sodium chloride solution is heated, the water will evaporate at 100 °C, leaving behind dry sodium chloride. This method effectively utilizes the differing boiling points to achieve separation.