A2 LEVEL CHEMISTRY

ACIDS, BASES AND BUFFERS

ASSESSED HOMEWORK

Answer all questions
Max 96 marks

	
Name		…………………………………………………………..

Mark		……../96	……....%	Grade ………

1.	Nitric acid, HNO3, is sold by a chemical supplier as a 65% solution, by mass. As supplied, each cubic decimetre of this nitric acid has a mass of 1400g.
	Calculate the pH of this solution.

pH = ....................................
[Total 3 marks]

2.	A student measured the pH of water as 7.0 at 25 °C. The student then warmed the water to 40 °C and measured the pH as 6.7.
	What do these results tell you about the tendency of water to ionise as it gets warmer?
Explain your reasoning in terms of equilibrium.
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[Total 2 marks]

3.	Phenol, C6H5OH, is a powerful disinfectant and antiseptic. Phenol is a weak Brønsted-Lowry acid.
	What is meant by the following terms;
(i)	a Brønsted-Lowry acid;
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[1]
(ii)	a weak acid?
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[1]
[Total 2 marks]

4.	When phenol is mixed with aqueous sodium hydroxide, an acid–base reaction takes place.
		C6H5OH(aq)	+	OH–(aq)		C6H5O–(aq)	+	H2O(l)
	......................		.....................		.......................		.....................
	In the available spaces,
•	label one conjugate acid–base pair as acid 1 and base 1,
•	label the other conjugate acid–base pair as acid 2 and base 2.
[Total 1 mark]

 5.	A solution of phenol in water has a concentration of 4.7 g dm–3.
(i)	Write an expression for the acid dissociation constant, Ka, of phenol.

[1]
(ii)	Calculate the pH of this solution of phenol. (Ka of phenol = 1.28 x 10-10 moldm-3)

[5]
[Total 6 marks]

6.	This question looks at two acids:
•	methanoic acid, HCOOH, a weak organic acid;
•	nitric acid, HNO3, a strong acid which can also act as a powerful oxidising agent.
	Methanoic acid is a weak BrØnsted-Lowry acid.
	Explain what is meant by a weak BrØnsted-Lowry acid.
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[Total 2 marks]

7.	Calculate the pH of a 0.025 mol dm–3 solution of methanoic acid. Show your working.
	For HCOOH, Ka = 1.58 x l0–4mol dm–3.

pH = ............................
[Total 3 marks]

8.	A student measured the pH of a solution of sodium hydroxide as 13.54 at 25 °C.
	Kw = 1.0 × 10–14 mol2 dm–6 at 25 °C.
 (i)	Write down an expression for the ionic product, Kw, for water.
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[1]
 (ii)	Calculate the concentration, in mol dm–3, of this solution of sodium hydroxide.

[2]
[Total 3 marks]

9.	Methanoic acid, HCOOH, is a weak organic acid which occurs naturally in ants and stinging nettles.
(a)	Use an equation for the dissociation of methanoic acid to show what is meant by a weak acid.
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[1]
(b)	A 1.50 × 10–2 mol dm–3 solution of HCOOH has [H+] = 1.55 × 10–3 mol dm–3.
(i)	Calculate the pH of this solution and give one reason why the pH scale is a more convenient measurement for measuring acid concentrations than [H+].
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[2]
(ii)	Write the expression for Ka for methanoic acid.

[1]
(iii)	Calculate the values of Ka and pKa for methanoic acid.

[3]
(iv)	Estimate the percentage of HCOOH molecules that have dissociated in this aqueous solution of methanoic acid.

[1]
[Total 8 marks]
10.	The Ka values for three acids are shown in the table below.
acid	Ka / mol dm–3	
	ethanoic acid	CH3COOH	1.70 ×10–5	
	phenol	C6H5OH	1.28 ×10–10	
	sulphurous acid	H2SO3	1.50 ×10–2	
(a)	What information is provided by Ka values?
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[1]
(b)	When sulphurous acid and ethanoic acid are mixed together, an acid-base reaction takes place.
		H2SO3(aq)	+	CH3COOH(aq)		HSO3–(aq)	+	CH3COOH2+(aq)
.......................		.......................		.......................		......................
(i)	In the spaces above
•	label one conjugate acid-base pair as acid 1 and base 1,
•	label the other conjugate acid-base pair as acid 2 and base 2.
[2]
(ii)	Predict and explain the acid-base reaction that would take place if ethanoic acid were mixed with phenol. Include an equation in your answer.
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[2]

(c)	The pH value of 0.0450 mol dm–3 hydrochloric acid is different from that of 0.0450 mol dm–3 ethanoic acid.
	Calculate the pH values of these two acids. Show all your working.

[5]
[Total 10 marks]

11.	When nitric acid is added to methanoic acid, the acid-base equilibrium below is set up.
HNO3 + HCOOH  NO3– + HCOOH2+
	Use this equilibrium to explain what is meant by the term conjugate acid-base pairs.
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[Total 3 marks]

12.	Hydroiodic acid, HI(aq), is a strong acid that is an aqueous solution of hydrogen iodide.
In the laboratory, hydroiodic acid can be prepared by the method below.
A mixture of 480 g of iodine and 600 cm3 of water was put into a flask. The mixture was stirred and hydrogen sulphide gas, H2S(g), was bubbled through for several hours.
The mixture became yellow as sulphur separated out. The sulphur was filtered off and the solution was purified by fractional distillation. A fraction of HI(aq) was collected containing 440 g of HI in a total volume of 750 cm3.	
(i)	Construct a balanced equation, with state symbols, for the preparation of hydroiodic acid.
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[2]
(ii)	Determine the percentage yield of hydroiodic acid.

[3]
(iii)	Calculate the pH of the hydroiodic acid fraction.

[2]
[Total 7 marks]

13.	Methanoic acid is a component of a buffer solution used in shampoos. The buffer solution can be made by mixing methanoic acid with another chemical.
(i)	State what is meant by a buffer solution.
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[1]
(ii)	Suggest a chemical that could be added to methanoic acid to prepare a buffer solution. Explain your answer.
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[2]
(iii)	What factors determine the pH of a buffer solution?
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[2]
[Total 5 marks]

14.	Hexylresorcinol is an antiseptic used in solutions for cleansing wounds and in mouthwashes and throat lozenges.
	The structure of hexylresorcinol is shown below.
		
	Identify a compound that could be added to hexylresorcinol to make a buffer solution.
Explain your answer.

[Total 2 marks]

15.	When dilute, nitric acid behaves as a typical acid.
	Write an equation for the reaction of nitric acid with limestone.
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[Total 2 marks]

16.	An excess of magnesium was added to 100 cm3 of 0.0450 mol dm–3 hydrochloric acid.
The same mass of magnesium was added to 100 cm3 of 0.0450 mol dm–3 ethanoic acid.
	Both reactions produced 54 cm3 of hydrogen gas, measured at room temperature and pressure, but the reaction with ethanoic acid took much longer to produce this gas volume.
	Explain why the reactions produced the same volume of a gas but at different rates.
	Use equations in your answer.
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[Total 4 marks]

17.	‘Superphosphate’ fertilisers contain calcium dihydrogenphosphate, Ca(H2PO4)2. This compound is one of the world's most important fertilisers. When dissolved in water, Ca(H2PO4)2 dissociates forming H2PO4– ions which are easily taken up by plants.
(a)	Calcium dihydrogenphosphate, Ca(H2PO4)2, is produced by treating rock phosphate, containing Ca3(PO4)2, with sulphuric acid, H2SO4.
	Write a balanced equation for this reaction.
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[1]
(b)	Aqueous H2PO4– ions can act as a weak acid.
	Write an equation to represent the dissociation of the H2PO4– ion.
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[1]
(c)	The H2PO4– ion can act as either an acid or a base.
(i)	State the formula of the conjugate base of H2PO4–.
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[1]
(ii)	State the formula of the conjugate acid of H2PO4–.
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[1]
(iii)	A solution of calcium dihydrogenphosphate, Ca(H2PO4)2, in water acts as a buffer solution.
	Suggest, with the aid of equations, how this buffering action takes place.
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[3]
[Total 7 marks]

18.	Chocolate mousse contains gelatine and a compound to promote fast setting of the mousse.
	Compound A is such a setting agent. It has two acidic hydrogen atoms per molecule and is one of the six acids listed below.
oxalic acid	HOOCCOOH
malonic acid	HOOCCH2COOH
succinic acid	HOOC(CH2)2COOH
glutaric acid	HOOC(CH2)3COOH
adipic acid	HOOC(CH2)4COOH
pimelic acid	HOOC(CH2)5COOH
	The student analysed a sample of compound A by titration.
	The student dissolved 2.82 g of compound A in water and made the solution up to 250 cm3 in a volumetric flask. He titrated 25.0 cm3 of this solution with 0.175 mol dm–3 NaOH.
	22.05 cm3 of NaOH were required for complete neutralisation.
	Use the results of the student’s analysis to identify compound A from the list above.
	Show all of your working.

[Total 5 marks]

19.	A student analysed an unsaturated carboxylic acid, A, using a titration procedure.
	The student dissolved 2.580 g of the compound in water and made the solution up to 250.0cm3. The student titrated 25.0 cm3 of this solution with 0.1263mol dm–3 NaOH. The volume of NaOH(aq) required to reach the end point was 23.75cm3.
	Each molecule of A has one acidic hydrogen atom and A behaves as a monoprotic (or monobasic) acid.
•	Calculate the molar mass of the unsaturated carboxylic acid.
•	Determine the molecular formula and possible displayed or skeletal formulae of the carboxylic acid.

[Total 8 marks]

20.	This question looks at different compounds used in medicine.
(a)	Nitrous oxide, N2O, is the gas used as a general anaesthetic.
(i)	What is the oxidation number of nitrogen in nitrous oxide?
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[1]
(ii)	Suggest a ‘dot-and-cross’ diagram for nitrous oxide. Show outer electrons only.
The sequence of atoms in a nitrous oxide molecule is N N O.

[1]
 (b)	The structure of the painkiller ibuprofen is shown below.

(i)	Determine the molecular formula of ibuprofen.

[1]
(ii)	Suggest a chemical that would react with a solution of ibuprofen to produce a gas.
Name the gas produced and write a balanced equation for the reaction.
chemical ...............................................................................................
gas ........................................................................................................
equation

[2]
 
(c)	Lidocaine, C13H20N2O2, is used as a local anaesthetic in dentistry. Lidocaine is administered by syringe as a solution containing 100 mg in 5.00 cm3.
	Calculate the concentration, in mol dm–3, of lidocaine in the syringe.

concentration = ......................mol dm–3
[3]
(d)	Eugenol is used as a painkiller in dentistry. It is an organic compound of C, H
and O.
	A sample of 1.394 g of eugenol was analysed by burning in oxygen to form 3.74 g of CO2 and 0.918 g of H2O. The relative molecular mass of eugenol was shown to be 164 using a mass spectrometer.
	Calculate the molecular formula of eugenol.

[5]
[Total 13 marks]