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Doc Brown's Chemistry
GCE AS A2 Level Chemistry**
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Advanced
Level Chemistry Revision on Titrations**
**GCE A
Level AS-A2 IB Acid-base and other non-redox volumetric titration
quantitative calculation questions PART 2 Questions 21 onwards**
In PART 2 of these A Level quantitative
chemistry calculation questions you have to work out the method a bit
more on your own! I have not broken the problems down into 'mini-steps' for
you, but request you use your experience from solving the PART 1 Questions.
I have also included some 'gas volume' and 'gravimetric analysis' questions
to provide more variation of quantitative analysis questions and
standardising hydrochloric acid calculations.
PART 1 Questions
* PART 1
Question Answers
PART 2 Question Answers * Redox
Titration Q's * Qualitative
Analysis
If you find these useful or
spot a silly error please
EMAIL
query?comment
ILLUSTRATIONS OF ACID-ALKALI
TITRATIONS and SIMPLE STARTER CALCULATIONS
**
**
I
DO MY BEST TO CHECK MY CALCULATIONS, as you yourself should do, BUT I
AM HUMAN! AND IF YOU THINK THERE IS A 'TYPO' or CALCULATION ERROR PLEASE
EMAIL ME ASAP TO SORT IT OUT!
**
**
**Relative atomic masses that may
be needed: H = 1, C = 12, O = 16, Na = 23, S = 32, Cl = 35.5, Ca = 40, Cu =
63.5, for questions 21 onwards**
**
I've tried to quote the data to the appropriate
significant figures and associated 'trailing zeros'.**
**
Q21 **Analysis of limewater. 25.0 cm^{3}
aliquots of a calcium hydroxide solution were titrated with 0.100 mol dm^{-3}
(0.100M) hydrochloric acid using phenolphthalein indicator. On average it took
10.50 cm^{3} of the acid to neutralise the limewater.
Calculate ...
(a) the molarity of the
calcium hydroxide solution
(b) the concentration of
calcium hydroxide in (i) g dm^{-3} and (ii) g/100 cm^{3}
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Q22** If it took 20.55 cm^{3} of 0.100 M HCl to
neutralise 25.0 cm^{3} of an NaOH solution, calculate the molarity of
the alkali.
**
Q23 **A standardised solution of sodium hydroxide had
a concentration of 0.1025 mol dm^{-3} (0.1025M). If 25.0 cm^{3} of a sulfuric acid
solution required 17.65 cm^{3} of the NaOH to neutralise it, calculate
the molarity of the acid.
**
Q24 **Citric acid has the formula
and is
tribasic acid, forming the tri-sodium salt
on
complete neutralisation with sodium hydroxide.
A 25.0 cm^{3} sample of a
concentrated citrus fruit cordial component (e.g. for lime juice), used in the
food & drinks industry, was diluted to 250 cm^{3} in a graduated
volumetric flask.
A 25.0 cm^{3} sample of this diluted solution,
required, on average, 22.5 cm^{3} of a standard 0.100 molar sodium
hydroxide solution using phenolphthalein indicator for the titration end-point.
Assuming all the acid in the cordial was citric acid, calculate the
concentration of the acid in g/dm^{3} in the original solution.
**
Q25 **4.28 g of a hydrated form of the salt sodium
sulphate, Na_{2}SO_{4}.xH_{2}O, was heated at 180^{o}C
until the remaining mass became constant at 1.89 g. Calculate x, the number of
molecules of 'water of crystallisation'.
**
Q26 **On reaction with dilute hydrochloric acid, 0.428
g of a group 2 metal (M) formed 75.0 cm^{3} of hydrogen gas at 298K/1 atm pressure.
If the molar volume of gas is 24.0 dm^{3} at 298K/1 atm, deduce the atomic mass of
the metal and identify it from the periodic table.
**
Q27 **To standardise a solution of hydrochloric acid,
it was titrated against accurate masses of anhydrous sodium carbonate (which
should be dried in an oven prior to use). e.g. 0.132 g of Na_{2}CO_{3}
required 24.8 cm^{3} of the hydrochloric acid for complete
neutralisation, calculate the molarity of the hydrochloric acid.
**
Q28 **Follows on from Q27. You can make up a solution
of sodium carbonate from which you pipette 25.0 cm^{3} aliquots to obtain an
accurate average titration value as an alternative to multiple weighings. 1.30g
of anhydrous sodium carbonate was dissolved in 250 cm^{3} of de-ionised water in a
volumetric flask. 25.0 cm^{3} aliquots were pipetted and titrated with a
hydrochloric acid solution (in the burette) to be standardised.
(a) Calculate the molarity of
the sodium carbonate solution.
(b) If the average titration
was 24.35 cm^{3} of hydrochloric acid, calculate the molarity of the
acid assuming the sodium carbonate is fully neutralised to sodium chloride.
**
Q29 **The purity of anhydrous sodium carbonate can be determined
by titration with standard hydrochloric solution. The following results were
obtained on titrating accurately weighed amounts of sodium carbonate with 1.00
mol dm^{-3} hydrochloric acid (1.00 M HCl) to complete neutralisation given
by the equation below.
**
Na**_{2}CO_{3}
+ 2HCl ==> 2NaCl + H_{2}O + CO_{2}
**Analysis Result** |
**Mass of Na**_{2}CO_{3} |
**titration** |
**(i)** |
**1.113 g** |
**20.95 cm**^{3} |
**(ii)** |
**1.092 g** |
**20.55 cm**^{3} |
**(iii)** |
**1.166 g** |
**22.00 cm**^{3} |
The titration values were
recorded to the nearest 0.05 cm^{3} for a
burette calibrated in 0.1 cm^{3} increments.
In each case calculate (a) the moles of
HCl used in each titration, (b) the moles of Na_{2}CO_{3} titrated based on
the moles of HCl used, (c) the mass of Na_{2}CO_{3} titrated
based on (b), and finally, (d) from your (b) and (c) calculations, calculate
the % purity of the anhydrous sodium carbonate, quoting what you think is
the most accurate analytical result.
**
Q30 **How to determine the water of crystallisation in hydrated sodium
carbonate crystals ('washing soda') by titration with standard hydrochloric acid
solution.
If 0.352g of hydrated sodium carbonate crystals, Na_{2}CO_{3}.xH_{2}O,
was titrated with 0.100 mol dm^{-3} (0.100M) standard hydrochloric
solution. If the titration value was 24.65 cm^{3},
calculate the moles of Na_{2}CO_{3}
titrated and hence deduce x, the number of molecules of water of crystallisation
in washing soda crystals.
**
Q31 **Blue hydrated copper(II) sulfate crystals have
the formula CuSO_{4}.xH_{2}O, where x is the number of molecules
of 'water of crystallization'.
If 3.33g of copper sulphate crystals where heated
at 150^{o}C until the residue of anhydrous copper sulphate remained constant at
2.12g.
(a) Calculate the % water of
crystallisation in the blue copper sulphate.
(b) From your answer to (a)
calculate the value of x in the formula of the blue crystals.
**Q32 **It is possible to analyse a
mixture of sodium hydrogencarbonate and anhydrous sodium carbonate, either as a
solid or solution, by titration with a standard acid solution e.g. 0.1 to 1.0
mol/dm^{3 }hydrochloric acid.
7.357 g of an solid anhydrous sodium
carbonate (Na_{2}CO_{3})/sodium hydrogencarbonate (NaHCO_{3})
mixture was dissolved in deionised water and made up to 250 cm^{3}
in a calibrated volumetric flask.
25.00 cm^{3} aliquots were
accurately pipetted into a conical flask and titrated with 0.5000 mol/dm^{3}
hydrochloric acid.
Using (1) phenolphthalein indicator
(pKind = 9.3), the average titration was 9.80 cm^{3} of the HCl.
With (2) methyl orange indicator (pKind =
3.7), the average titration was 24.75 cm^{3} HCl.
**(a)(i)** On adding the acid to the
alkaline carbonate mixture solution, what are the colour changes for the
end-points of titration (1) and (2)?
**(a)(ii)** Give the equations for
what happens in each titration.
**(b)(i)** Calculate the moles of Na_{2}CO_{3}
in the prepared solution and calculate its molarity.
**(b)(ii)** Calculate the mass of
sodium carbonate in the solution and hence the percentage Na_{2}CO_{3}
in the original solid mixture.
**Extra calculations** for further
practice and will partly help you to solve Q33
**(c)(i)** In the titrations, what
total volume of the HCl was used to neutralise the Na_{2}CO_{3}?
**(c)(ii)** In titration (2) what
volume of HCl was used to neutralise the NaHCO_{3} in the original
mixture?
**(c)(iii)** Calculate the number of
moles of NaHCO_{3} in the 25.0 cm^{3} aliquot at the start,
and the total moles in the prepared solution.
**(c)(iv)** Calculate the mass off
NaHCO_{3} in the prepared solution, and hence its % in the original
solid mixture.
If your answer to **(b)(ii) + (c)(iv)**
do not add up to **100%**, you have made an error somewhere!
**Q33**** **
Sodium hydroxide solution can be used to absorb carbon dioxide gas.
2 dm^{3} (2 litres) of a sodium
hydroxide solution was used to cleanse a gas mixture of carbon dioxide. The
resulting solution X now consists of a mixture of unreacted sodium hydroxide and
sodium carbonate.
Solution X was titrated with standard 1.000 mol/dm^{3}
hydrochloric acid ...
(1) using phenolphthalein
indicator.
(2) using methyl orange indicator and
From these titration values you can calculate the molarity of
unreacted sodium hydroxide, the molarity of the sodium carbonate formed and how
much carbon dioxide gas was absorbed.
Using 25.00 cm^{3} pipetted
aliquots of solution X, on average ...
titration (1) with phenolphthalein gave a value of 12.45
cm^{3} of 1.000 mol/dm^{3} HCl
titration (2) with methyl orange gave a
total value of 22.55
cm^{3} of 1.000 mol/dm^{3} HCl
(a) After some research, briefly explain
why titration ...
(1) using phenolphthalein indicator
corresponds to the end-point of the titration-half-neutralisation of
NaOH and Na_{2}CO_{3}
to NaHCO_{3}
(2) using methyl orange indicator
corresponds to the end-point of the titration of the complete neutralisation of NaOH
plus all of Na_{2}CO_{3}
How you can do the analysis with a
single titration?
(b) Calculate the molarity of the sodium
carbonate formed.
(c) Calculate the molarity of the
unreacted sodium hydroxide.
(d) Calculate the volume of carbon
dioxide gas absorbed (assume molar gas volume = 24 dm^{3} at RTP).
**
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PART 1 Questions
* PART 1
Question Answers *
PART 2 Question Answers * Redox
Titration Q's
**
**
I
DO MY BEST TO CHECK MY CALCULATIONS, as you yourself should do, BUT I
AM HUMAN! AND IF YOU THINK THERE IS A 'TYPO' or CALCULATION ERROR PLEASE
EMAIL ME ASAP TO SORT IT OUT!
ILLUSTRATIONS OF ACID-ALKALI
TITRATIONS and SIMPLE STARTER CALCULATIONS
Advanced Level
ORGANIC CHEMISTRY REVISION
STUDY NOTES
Advanced Level INORGANIC CHEMISTRY REVISION
STUDY NOTES
Advanced Level PHYSICAL-THEORETICAL REVISION
STUDY NOTES
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