Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 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: Volumetric acid-base analysis
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-02-2015-91161-02-5417
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 2.1B v2 for Achievement Standards 91161
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Internal Assessment Resource
Achievement Standard Chemistry 91161: Carry out quantitative
analysis
Resource reference: Chemistry 2.1B v2
Resource title: Volumetric acid-base analysis
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.
Context/setting
This activity requires students to carry out a volumetric acid-base analysis to collect
primary data and process primary and secondary data to solve simple quantitative
problems. Students will perform an acid-base titration to standardise a solution of
acid, and calculate the concentration of the acid. They will then use provided data to
solve simple quantitative problems.
Although the context for this assessment is the common household cleaner sodium
carbonate, the task is not specific to sodium carbonate and may be modified as
appropriate for your setting, for example, vinegar, sulphuric acid, or caustic soda.
Conditions
Students should work individually on this assessment, in class. This assessment will
require multiple sessions, for a total of approximately 2 hours of class time. Choose
where to split the assessment resource and update the student instructions
accordingly. The suggested time allowed is 1 hour for the practical part and 1 hour
for the written part.
You must accurately determine the solutions for the titration in advance (one of the
requirements of the standard is accuracy). Carry out the procedure prior to the
assessment and determine the expected titre for the sodium carbonate sample used.
Give the students the concentration of the sodium carbonate solution (to 3 significant
figures).
Resource requirements
Provide students with the following equipment:

20.0 or 25.0 mL pipette

burette
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
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
3 conical flasks

standard hydrochloric acid solution (approximately 0.100 mol L1)

sodium carbonate solution made by adding approximately 5.3 g anhydrous
Na2CO3 to 1 L water

distilled water

methyl orange indicator

wash bottle.
Additional information
When work is submitted to NZQA for external moderation, clearly identify the
expected titre of sodium hydroxide solution needed in the titration and the
corresponding concentration of hydrochloric acid.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
PAGE FOR STUDENT USE
Internal Assessment Resource
Achievement Standard Chemistry 91161: Carry out quantitative
analysis
Resource reference: Chemistry 2.1B v2
Resource title: Volumetric acid-base analysis
Credits: 4
Achievement
Achievement with Merit
Carry out quantitative
analysis.
Carry out in-depth
quantitative analysis.
Achievement with
Excellence
Carry out comprehensive
quantitative analysis.
Student instructions
Introduction
In this assessment, you will use quantitative analysis to determine the concentration
of an unknown solution of hydrochloric acid. You will carry out an acid-base titration
to standardise the hydrochloric acid using a known concentration of sodium
carbonate. You will then solve simple quantitative problems relating to some
household cleaners. You will be assessed on the accuracy of your titration data and
calculations.
Hydrochloric acid is highly reactive; working with unknown concentrations of HCL can
be dangerous. Quantitative analysis of the reaction between sodium carbonate and
hydrochloric acid can help you determine the concentration of the hydrochloric acid
solution.
Sodium carbonate has been used for generations as a household cleaner; it is now
regaining its popularity as a ‘green’ cleaning substance.
Na2CO3 reacts with hydrochloric acid, HCL. The equation for this reaction is
Na2CO3 + 2HCl

2NaCl + CO2 + H2O
Teacher note: This task is not specific to sodium carbonate and hydrochloric
acid. You many modify the context as appropriate for your setting, for example,
vinegar, sulphuric acid, or caustic soda.
All work should be done individually, in class. You may need more than one session
to complete this assessment, depending on the availability of equipment and the
length of each class period; confirm the timeframe with your teacher.
Teacher note: You should confirm the assessment timeframes with your students
before conducting the assessment.
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
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Task
Part A: Practical activity
Working on your own, use the equipment provided by your teacher to titrate 25.0 mL
of sodium carbonate solution of known concentration with hydrochloric acid until you
have at least three concordant titres. Record your titre volumes to 2 decimal places.
Using your primary data (the titre volumes) and your knowledge of stoichiometry (the
relationships between reactants and products), calculate the concentration of
hydrochloric acid in the unknown solution. Show all of your working, and give
answers to three significant figures, with units.
You will need:

25.0 mL pipette

burette

3 conical flasks

diluted hydrochloric acid solution

sodium carbonate solution (Concentration = _______mol L-1)

distilled water

methyl orange indicator

wash bottle.
Results
Teacher note: Leave sufficient space for students to write their answers.
Calculations
Calculate the average volume for hydrochloric acid.
Teacher note: Leave sufficient space for students to write their answers.
Use the known concentration of sodium carbonate to calculate the amount, in moles,
of hydrochloric acid.
Teacher note: Leave sufficient space for students to write their answers.
Calculate the concentration of hydrochloric acid in the unknown solution used in the
titration. Give your answer to three significant figures.
Teacher note: Leave sufficient space for students to write their answers.
This resource is copyright © Crown 2015
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
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Part B: Written activity
Tile cleaners contain hydrogen peroxide. Hydrogen peroxide is 94.12% oxygen and
5.88% hydrogen. It has a molar mass of 34.0 g mol-1. Use the following molar
masses to calculate both the empirical formula and the molecular formula of
hydrogen peroxide.
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
Teacher note: Leave sufficient space for students to write their answers.
Citric acid, formula C6H8O7, can be used to soften water which makes it useful in
soaps and laundry detergents. Calculate the percentage composition of citric acid.
Teacher note: Leave sufficient space for students to write their answers.
Citric acid reacts with sodium hydroxide. The equation for the reaction is:
C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
Calculate the mass of sodium citrate, C6H5O7Na3, which is formed when 10.0g of
sodium hydroxide reacts with citric acid.
Teacher note: Leave sufficient space for students to write their answers.
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
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Assessment schedule: Chemistry 2.1B Volumetric acid-base analysis
Required information from the teacher: Standard sodium carbonate solution concentration n = [insert value, for example 0.0495] mol L1,
Expected titre = [insert value, for example 22.70] mL.
Evidence/Judgements for Achievement
Evidence/Judgements for Achievement with
Merit
Evidence/Judgements for Achievement with
Excellence
Student correctly:
 records initial and final volumes of
hydrochloric acid used in the titration – at
least three titres fall within a range of 0.4
mL
 calculates average titre value to within 0.8
mL of the expected titre, using all titre
values
 uses the relationships n=m/M and c=n/V to
calculate one variable given the other two,
for example, the student determines the
number of moles of sodium carbonate
based on the given concentration and
measured volume: n(Na2CO3) = 0.0495 x
0.025 = 1.24 x 10-3 mol.
A minor error in calculation is allowed as long
as the procedure is appropriate, for example, an
incorrect conversion from mL to L. Units and 3
significant figures are not required for achieved,
however, titre volumes should be given to at
least 2 decimal places.
Student correctly:
 records initial and final volumes of
hydrochloric acid used in the titration – at
least three titres fall within a range of 0.4
mL
 calculates average titre value to within 0.5
mL of the expected titre, using only
concordant titre values
 uses the relationships n=m/M and c=n/V to
solve problems involving at least 2 steps,
for example, the student determines the
number of moles of hydrochloric acid based
on the measured titre volume: n(Na2CO3) =
0.0495 x 0.025 = 1.24 x 10-3 mol.
n(Na2CO3) : n(HCl) = 1:2, so n(HCl) = 2.47x
10-3 mol.
Student correctly:
 records initial and final volumes of
hydrochloric acid used in the titration – at
least three titres fall within a range of 0.2
mL
 calculates average titre value to within 0.2
mL of the expected titre, using only
concordant titre values
 uses the relationships n=m/M and c=n/V to
solve stoichiometric problems involving
more than 2 steps, for example, the student
determines the concentration of
hydrochloric acid: n(Na2CO3) = 0.0495 x
0.025 = 1.24 x 10-3 mol. n(Na2CO3) : n(HCl)
= 1:2, so n(HCl) = 2.47x 10-3 mol. c(HCL) =
2.47x 10-3 mol / 0.0227 L = 0.109 mol L-1.
Units and 3 significant figures are required for
excellence.
 Mass of sodium citrate correctly
calculated
Example:
n(NaOH) = 10/40 = 0.250 mol
n(NaOH) : n(C6H5O7Na3) = 3:1
n(C6H5O7Na3) = 0.250/3 = 0.0833 mol
m(C6H5O7Na3) = 0.0833 x 258 = 21.5g
 Consistent use of significant figures and
correct units
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c(HCl) = 2.47X10-3/0.0227 = 0.109 mol.L-1
Units and 3 significant figures are not required
for merit. Titre volumes should be given to 2
decimal places.
 Empirical and molecular formulae are
calculated
Example:
n(H) = 5.88/1.00 = 5.88 mol
n(O) = 94.12/16.0 = 5.88 mol
empirical formula = HO
M(HO) = 17g.mol-1 therefore
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Internal assessment resource Chemistry 2.1B v2 for Achievement Standards 91161
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Molecular formula = H2O2
 Percentage composition is calculated
Example:
%C = 72.0/192 (x100) = 37.5%
%H = 8.0/192 (x100) = 4.20%
%O = 112/192 (x100) = 58.3%
 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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