Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR TEACHER USE
NZQA
Approved
Internal Assessment Resource
Chemistry Level 2
This resource supports assessment against:
Achievement Standard 91161 version 2
Carry out quantitative analysis
Resource title: Sour grapes and rhubarb
4 credits
This resource:

Clarifies the requirements of the standard

Supports good assessment practice

Should be subjected to the school’s usual assessment quality assurance
process

Should be modified to make the context relevant to students in their school
environment and ensure that submitted evidence is authentic
Date version published by
Ministry of Education
February 2015 Version 2
Quality assurance status
These materials have been quality assured by NZQA.
To support internal assessment from 2015
NZQA Approved number: A-A-02-2015-91161-02-5416
Authenticity of evidence
Teachers must manage authenticity for any assessment
from a public source, because students may have access
to the assessment schedule or student exemplar material.
Using this assessment resource without modification may
mean that students’ work is not authentic. The teacher
may need to change figures, measurements or data
sources or set a different context or topic to be
investigated or a different text to read or perform.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
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Internal Assessment Resource
Achievement Standard Chemistry 91161: Carry out quantitative
analysis
Resource reference: Chemistry 2.1A v2
Resource title: Sour grapes and rhubarb
Credits: 4
Teacher guidelines
The following guidelines are designed to ensure that teachers can carry out valid and
consistent assessment using this internal assessment resource.
Teachers need to be very familiar with the outcome being assessed by the
Achievement Standard Chemistry 91161. The achievement criteria and the
explanatory notes contain information, definitions, and requirements that are crucial
when interpreting the standard and assessing students against it.
Additional information
The teacher should carry out the procedure prior to the student practical assessment
to determine the expected titre for the vinegar sample used.
Context/setting
This activity requires students to carry out a volumetric acid-base analysis to collect
primary data, and to process primary and secondary data to solve simple quantitative
problems.
The students will use volumetric analysis to determine the concentration of ethanoic
acid in a diluted vinegar solution. They will then use provided data relating to oxalic
acid (found in rhubarb) to solve simple quantitative problems.
You may change the pipette size used depending on what is available. 20 or 25 mL
is preferable. The concentration of the sodium hydroxide solution should be
approximately 0.100 mol L-1 and must be accurately known.
Conditions
This individual assessment task is made up of one practical part and one written part.
The suggested time allowed is 1 hour for the practical part and 1 hour for the written
part.
You may choose where to split the assessment resource time-wise and alter the
student instructions accordingly. One possible stopping point could be after the
experimental results have been obtained, leaving all calculations to the second
period.
Resource requirements
Students require access to:

20.0 or 25.0 mL pipette (the task and schedule is based on a 25.0 mL pipette)
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR TEACHER USE

burette

3 conical flasks

diluted household vinegar sample (diluted 1 in 10)

standard sodium hydroxide solution (approximately 0.100 mol L-1)

distilled water

phenolphthalein indicator

wash bottle.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR STUDENT USE
Internal Assessment Resource
Achievement Standard Chemistry 91161: Carry out quantitative
analysis
Resource reference: Chemistry 2.1A v2
Resource title: Sour grapes and rhubarb
Credits: 4
Achievement
Carry out quantitative
analysis
Achievement with Merit
Carry out in-depth
quantitative analysis
Achievement with
Excellence
Carry out comprehensive
quantitative analysis
Student instructions
Introduction
This assessment resource requires you to carry out a volumetric acid-base analysis
by titrating a diluted vinegar solution. You are also required to use provided data
relating to oxalic acid (found in rhubarb) to solve simple quantitative problems.
There are two parts to the task: a practical part and a written part. You will be
assessed on both parts. Record your results and calculations.
This is an individual assessment. You have one hour for each part.
Make sure you:

show all working

give your answer to three significant figures

use appropriate units.
Task
Vinegar has been known since ancient times. It is produced when wine goes sour
and is often used as an ingredient in salad dressings and pickles because of its
acidic nature. The active ingredient is ethanoic acid.
Ethanoic acid is one of the simplest carboxylic acids. It is an important chemical
reagent and industrial chemical, used in the production of soft drink bottles, PVA
glue, and synthetic fibres and fabrics. In the food industry, ethanoic acid is used
under the food additive code E260 as an acidity regulator.
Oxalic acid is another commonly occurring carboxylic acid which is found in leafy
plants such as rhubarb. Oxalic acid is toxic to humans in large amounts and can
cause kidney and bladder stones.
Part A: Practical activity
Purpose
You will need to carry out a titration to analyse the diluted household vinegar solution
provided by your teacher. You will need:
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR STUDENT USE

25.0 mL pipette

burette

3 conical flasks

diluted household vinegar solution (diluted 1 in 10)

distilled water

phenolphthalein indicator

wash bottle

standard sodium hydroxide solution (Concentration = _____________ mol L-1).
Method

Titrate 25.0 mL samples of the diluted vinegar solution with the standard sodium
hydroxide solution using phenolphthalein indicator

Repeat the titration until you have concordant results (a minimum of three
repetitions is required)

Record all of your burette readings in a systematic format that clearly indicates
the volume of sodium hydroxide used in each titration.
Results
Leave sufficient space for students to write their answers.
Calculations
Calculate the average titre volume for the sodium hydroxide.
Leave sufficient space for students to write their answers.
Use the known concentration of sodium hydroxide to calculate the amount, in moles,
of sodium hydroxide.
Leave sufficient space for students to write their answers.
The balanced equation for the reaction occurring in the titration is:
CH3COOH + NaOH

CH3COONa + H2O
Calculate the concentration of ethanoic acid in the diluted vinegar solution used in
the titration. Give your answer to three significant figures.
Leave sufficient space for students to write their answers.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR STUDENT USE
The vinegar solution was diluted 1:10. Calculate the concentration of the original
vinegar solution.
Leave sufficient space for students to write their answers.
Part B: Written activity
Rhubarb plants contain 1.36% oxalic acid. Oxalic acid is 26.7% carbon, 2.22%
hydrogen and 71.1% oxygen, and has a molar mass of 90.0 g mol1. Use the
following molar masses to calculate both the empirical formula and the molecular
formula of oxalic acid:
M(C) = 12.0 g mol1 M(H) = 1.00 g mol1 M(O) = 16.0 g mol1 M(Na) = 23.0 g mol1
Leave sufficient space for students to write their answers.
Rhubarb also contains 2.15% malic acid, formula C4H6O5. Calculate the percentage
composition of malic acid.
Leave sufficient space for students to write their answers.
Malic acid reacts with sodium hydroxide. The equation for the reaction is:
C4H6O5 + 2NaOH

C4H4O5Na2 + 2H2O
Calculate the maximum mass of sodium malate, C4H4O5Na2, which could be made
from 15.0g of sodium hydroxide.
Leave sufficient space for students to write their answers.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
PAGE FOR TEACHER USE
Assessment schedule: Chemistry 91161 Sour grapes and rhubarb
Example of required information from the teacher: standard sodium hydroxide solution concentration = 0.105 mol L1, expected titre = 23.70
mL.
Evidence/Judgements for Achievement







Initial and final volumes of sodium hydroxide
used in the titration are recorded.
Titre volumes have been calculated.
At least three titres must fall within a range of
0.4 mL.
The average titre value is calculated and is
within 0.8 mL of the expected outcome.
Using titration data calculation of the
concentration of an unknown solution is
carried out using appropriate procedure.
All titre values may be used to determine the
average titre.
A minor error in the calculation is allowed but
the concentration determined must be a
sensible one.
Example:
Incorrect substitution of a numerical error or
incorrect conversion of volumes such as mL to L.




There is no penalty for omitting units or
inappropriate use of significant figures.
Amount in moles of each element correct.
Percentage composition process correct but
incorrect molar mass of C4H6O5 is calculated.
Molar mass of sodium hydroxide (40.0) and
sodium malate (178) are correctly calculated.
This resource is copyright © Crown 2015
Evidence/Judgements for Achievement
with Merit





Initial and final volumes of sodium hydroxide
used in the titration are recorded.
Titre volumes have been calculated.
At least three titres must fall within a range of
0.4 mL.
The average titre value is calculated and is
within 0.5 mL of the expected outcome.
Using only concordant titres the concentration
of the diluted solution is correctly calculated.
10-3
n(NaOH) = 0.105 x 0.02370 = 2.49 x
n(CH3COOH) : n(NaOH) = 1:1
so n(CH3COOH) = 2.49 x 10-3 mol
c(CH3COOH) = 2.49 x 10-3 / 0.0250 mL
= 0.0995 mol L-1








Example:

Evidence/Judgements for Achievement
with Excellence
mol
There is no penalty for omitting units or
inappropriate use of significant figures.
The concentration of the original solution is
calculated.
e.g. 0.0995 mol L-1 x 10 = 0.995 mol L-1
Empirical and molecular formula is calculated

Initial and final volumes of sodium hydroxide
used in the titration are recorded.
Titre volumes have been calculated.
At least three titres must fall within a range of
0.2 mL.
The average titre value is calculated and is
within 0.2 mL of the expected outcome.
Using only concordant titres the concentration
of the unknown solution is correctly calculated.
The concentrations calculated must have
correct units and three significant figures.
Mass of sodium malate correctly calculated.
Example:
n(NaOH) = 15.0 / 40 = 0.375 mol
n(NaOH) : n(C4H4O5Na2) = 2:1
so n(C4H4O5Na2) = 0.375 /2
= 0.188 mol
m(C4H4O5Na2) = 0.1875 x 178
= 33.4g

Consistent use of correct units and three
significant figures.
Example:
n(C) = 26.7/12.0 = 2.23 mol
n(H) = 2.22/1.00 = 2.22 mol
n(O) = 71.1/16.0 = 4.44 mol
empirical formula CHO2
M(empirical formula) = 45 g mol1 therefore
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Internal assessment resource Chemistry 2.1A v2 for Achievement Standard 91161
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molecular formula = C2H2O4

Percentage composition is calculated.
Example:
%C = 48.0 / 134 (x100) = 35.8%
%H = 6.00 / 134 (x100) = 4.48%
%O = 80.0 / 134 (x100) = 59.7%

Two correct steps of the calculation are
shown.
Final grades will be decided using professional judgement based on a holistic examination of the evidence provided against the criteria in the
Achievement Standard.
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