Chemistry - annotated exemplar level 2 AS91161

Carry out quantitative analysis (2.1)

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This annotated exemplar is intended for teacher use only. The student work shown does not always represent a complete sample of what is required. Selected extracts are used, focused on the grade boundaries, in order to assist assessors to make judgements at the national standard.

Low Excellence

Commentary
Student work extract

Student 1 (PDF, 112KB)

For Excellence, the student needs to carry out comprehensive quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.2 mL; the average titre value must be within 0.2 mL of the expected outcome
  • solving quantitative problems that involve more than two steps
  • the use of stoichiometric principles.

Answers to calculations must demonstrate correct units and appropriate use of significant figures.

The student has collected titration data containing three titre values within 0.2 mL of each other and the average titre value is within 0.2 mL of the expected outcome (1). Quantitative problems that involve more than two steps and the use of stoichiometric principles, involving n=m/M and c=n/V are solved using appropriate procedures (2).

For a more secure Excellence, the student could give all final answers to three significant figures and use correct units (3).

High Merit

Commentary
Student work extract

Student 2 (PDF, 127KB)

For Merit, the student needs to carry out in-depth quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.4 mL; the average titre value must be within 0.5 mL of the expected outcome
  • solving quantitative problems that involve at least two steps and require application of relationships such as n=m/M and c=n/V.

Titration calculations must be carried out correctly using only concordant titre values.

The student has collected titration data that contains three titre values that fall within 0.2 mL of each other but the average titre value is within 0.5 mL of the expected outcome (1). Quantitative problems that involve at least two steps, involving n=m/M and c=n/V have been solved using appropriate procedures (2).

To reach Excellence, the student could give all final answers to three significant figures (3) and solve problems involving more than two steps and using stoichiometric principles (4).

Low Merit

Commentary
Student work extract

Student 3 (PDF, 111KB)

For Merit, the student needs to carry out in-depth quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.4 mL; the average titre value must be within 0.5 mL of the expected outcome
  • solving quantitative problems that involve at least two steps and require application of relationships such as n=m/M and c=n/V.

Titration calculations must be carried out correctly using only concordant titre values.

The student has collected titration data that contains three titre values that fall within 0.2 mL of each other but the average titre value is within 0.5 mL of the expected outcome (1). Quantitative problems involving c=n/V have been solved correctly (2). A quantitative problem involving n=m/M has been solved (3).

For a more secure Merit, the student could correctly solve a quantitative problem that uses the relationship n=m/M involving at least two steps (3).

High Achieved

Commentary
Student work extract

Student 4 (PDF, 111KB)

For Achieved, the student needs to carry out quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.4 mL; the average titre value must be within 0.8 mL of the expected outcome
  • solving quantitative problems that use the relationships n=m/M and c=n/V to calculate one variable given the other two (the relationships are not given).

The student has collected titration data that contains three titre values that fall within 0.4 mL of each other and the average titre value is within 0.8 mL of the expected outcome (1). A quantitative problem involving c=n/V (titration problem) has been solved (2). A Quantitative problem using n=m/M is calculated correctly (3).

To reach Merit, the student could collect titration data for which the average titre value is within 0.5 mL of the expected outcome (1) and correctly calculate a two-step quantitative problem that uses the relationship n=m/M (4).

Low Achieved

Commentary
Student work extract

Student 5 (PDF, 111KB)

For Achieved, the student needs to carry out quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.4 mL; the average titre value must be within 0.8 mL of the expected outcome
  • solving quantitative problems that use the relationships n=m/M and c=n/V to calculate one variable given the other two (the relationships are not given).

The student has collected titration data that contains three titre values that fall within 0.4 mL of each other and the average titre value is within 0.8 mL of the expected outcome (1). Titration calculation is carried out correctly (2). Part of a quantitative problem using n=m/M has been solved correctly (3).

For a more secure Achieved, the student could correctly solve an entire quantitative problem using n=m/M.

High Not Achieved

Commentary
Student work extract

Student 6 (PDF, 128KB)

For Achieved, the student needs to carry out quantitative analysis.

This involves:

  • collecting titration data that contains at least three titre values that fall within a range of 0.4 mL; the average titre value must be within 0.8 mL of the expected outcome
  • solving quantitative problems that use the relationships n=m/M and c=n/V to calculate one variable given the other two (the relationships are not given).

The student has collected titration data that contains three titre values and the average titre value is within 0.8 mL of the expected outcome (1). Titration calculation is carried out correctly (2). A quantitative problem using n=m/M has been solved (3).

To reach Achieved, the student could collect titration data that contain at least three titre values that fall within a range of 0.4 mL (4).

 
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