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Chemistry Module 2 Assignment 2

Ions in solution, acids and bases and volumetric analysis

Answer all the following question ON PAPER and submit to your lecturer with a cover sheet in your class in the

Week Beginning 28thJan 2013.

The accuracy of your written English will be taken into account in marking.

1.(a)Identify the spectator ions in the following equation[1 mark]

Cl2 (aq) + 2 K+ (aq) + 2 Br (aq)  Br2 (aq) + 2 K+ (aq) + 2 Cl (aq)

(b)Rewrite the equationto give the net ionic reaction.[1 mark]

Use the solubility guidelines below to help you answer question 2

Cation / Anion
Cl. Br, I / SO42 / CO32 / OH / NO3
Na+, K+ / S / S / S / S / S
Mg2+ / S / S / X / X / S
Ca2+ / S / X / X / Sparingly / S
Ba2+ / S / X / X / Sparingly / S
Al3+ / S / S / - / X / S
Zn2+ / S / S / X / X / S
Pb2+ / X / X / X / X / S
Ag+ / X / Sparingly / X / - / S

S = Soluble, X = insoluble, - = no salt

  1. An unlabelled bottle contained a solution of one of the following: AgNO3, CaCl2 or Al2(SO4)3. In order to establish the identity of this solution known samples of AgNO3, CaCl2 and Al2(SO4)3 were reacted separately with Ba(NO3)3 and then with NaCl.

(a)Copy the table below and complete it to indicate whether or not a precipitate was obtained in these reactions. Write the formula of any precipitates produced. [5marks]

Compound / Ba(NO3)2 result / NaCl result
AgNO3 (aq)
CaCl2 (aq)
Al2(SO4)3 (aq)

(b)Write (i) a balanced equation and (ii) a net ionic equation for the reactions that occur. Include state symbols. Identify the spectator ion(s) in the solutions. [8 marks]

(c)What difference (if any) would there be in the reactions if sodium chloride were to be replaced by potassium chloride? [1 mark]

  1. Magnesium carbonate, magnesium oxide and magnesium hydroxide are all white solids that react with acidic solutions.

(a)Write a balanced equation and an ionic equation for the reaction that occurs when each substance reacts with a hydrochloric acid solution. Include state symbols. [3  3 marks]

(b)By observing the reactions in part (a) could you distinguish any of the three magnesium substances from the other two? If so, how? [2 marks]

(c)If excess HCl (aq) is added, would the clear solution left behind after each reaction is complete contain the same or different ions in each case? Identify the ions present. [2 marks]

4.Nitrous acid engages in a proton-transfer reaction with the methanoate ion, HCO2 :

HNO2(aq) + HCO2 (aq) NO2(aq) + HCO2H (aq)

(a)For the forward reaction identify the acid and base.[2 marks]

(b)Identify the acid and base for the reverse reaction.[2 marks]

(c)Identify the conjugate of HNO2. Is it the conjugate acid or the conjugate base? [1 mark]

(d)Identify the other conjugate acid-base pair and classify each species as the acid or the base. [2 marks]

  1. Calculate

(a) the number of grams of solute in 250 cm3 of 0.175 mol dm3 KBr;

(b)the molar concentration of a solution containing 14.75 g of Ca(NO3)2 in 1.375 dm3;

(c)the volume of 1.50 mol dm3 Na3PO4, in cm3, that contains 2.50 g of solute.

Express your answer to the correct number of significant figures.[3  2 marks]

6.A solution of NaCl has a concentration of 0.100 mol dm3.

(a)What is the concentration of the NaCl solution in ppm (mg dm3)?[2 marks]

(b)What are the concentrations in ppm of the Na+ and Cl ions?[3 marks]

  1. A solution of 0.204 mol dm-3 sodium hydroxide (NaOH) was used to neutralise 50.0 cm3 phosphoric acid (H3PO4) and 16.4 cm3 of sodium hydroxide solution was required to reach the end-point.

(a)Write a balanced equation for the reaction involved.[2 marks]

(b)Calculate the number of moles of sodium hydroxide used in the titration.[1 mark]

(c)Calculate the number of moles of sodium phosphate used in the titration.[2 marks]

(d)Calculate the concentration of the phosphoric acid solution?[1 mark]

8.(a)In order to standardise a solution of sodium hydroxide, a chemist first prepared a solution of ethanedioic acid–2–water, HOOCCOOH2H2O, by dissolving 14.6 g of ethanedioic acid–2–water in water and making the solution up to 250 cm3 in a graduated flask. He then pipetted 25.0 cm3 of this solution into a conical flask, added phenolphthalein solution as indicator, and titrated it against the sodium hydroxide solution: 24.1 cm3 of the latter were required.

(i)Draw a display structure for ethanedioic acid[2marks]

(ii)Hence write a balanced equation for the reaction between the acid (HOOCCOOH) and sodium hydroxide (NaOH).

[2mark]

(iii)Calculate the number of moles of the acid, HOOCCOOH2H2O in 25.0 cm3[2 marks]

(iv)Calculate the concentration of the sodium hydroxide solution.[2 marks]

(b)The chemist then used the standardised sodium hydroxide to estimate the concentration of sulphuric acid from a car battery. He found that 2.00 cm3 of battery acid were neutralised by 20.7 cm3 of the sodium hydroxide solution. Calculate the concentration of sulphuric acid in the battery acid, first in mol per dm3 and then in grams per dm3. [5 marks]

(c)Why does the student have to calculate the concentration of the sodium hydroxide using this titration method rather than just weighing out a given amount and dissolving it in a known volume of water? [2 marks]

Total Marks = 68