Wednesday, March 1, 2017

CSEC Chemistry January 2017 P2 - Full Solution

Answers to remaining questions will be added as quickly as we can type and upload.

Question 1

1(a)  A standard solution is one whose concentration is accurately known.

1(b)    Mass of sample = 3.16 g

1(c)(iFinal volumes
  
           Titration 1: 25.60 cm3
           Titration 2: 26.30 cm3
           Titration 3: 24.50 cm3

1(c)(ii)    Volume used:
  
         Titration 1: 24.60 cm3
         Titration 2: 24.25 cm3
         Titration 3: 24.35 cm3

1(c)(iii)  According to the wording in this question, the student is required to use values from all three titrations to calculate the average volume used. This   however, is not what should actually be done in the lab. Correct treatment of this data says that you should ignore Titration 1 and find the average of   volumes used in Titrations 2 and 3. Both versions are shown here. The one   in which all three values were used, is in red and the other in green. Please   ask your teacher at school whether they subscribe to the literal interpretation of this question (in red) or not.
             
             Average volume used = 24.40 cm3
             Average volume used = 24.30 cm3

1(c)(iv)  0.000244 mol
              0.000243 mol

1(d)(i)  0.001220 mol
             0.001215 mol

1(d)(ii)  0.01220 mol
              0.01215 mol

1(e)  1.85 g
        1.85 g

1(f)  1.31 g
        1.31 g

1(g)  0.07278 mol
         0.07278 mol

1(h)  5.97 which is 6 to the nearest whole number.
         5.97 which is 6 to the nearest whole number.

1(i)  The solution goes from pale green to very pale pink/purple.

1(j)  The unreacted potassium permanganate present show that the end point has been reached because its colour does not disappear.

1(k)(i)  Dirty green precipitate is formed.
            Precipitate is in insoluble in excess aqueous NaOH.

1(k)(ii)  Precipitate slowly changes colour from dirty green to dirty brown/rust   brown.

1(k)(iii)  White precipitate formed.
               Precipitate is insoluble in dilute nitric acid.

Question 2

2(a)(i)  Electrolysis is defined as the chemical decomposition of an electrolyte by the passage of an electric current.

2(a)(ii)  Electroplating is the deposition of a metal on the cathode of an electrolytic cell, via the passage of an electric current, if the right combination of   electrolyte and electrodes (or electrode system) is used.

2(a)(iii)  The extraction of reactive metals e.g. Na and Al from their ores/compounds.
 
               Extraction of active non-metals e.g. the halogens, like Cl2 from NaCl(aqusing the diaphragm cell.

              Anodizing of aluminium.

2(b)(i       


              The electrodes should be made of graphite.

2(b)(ii)  Equation at anode: 2Br-(l) Br2(g) + 2e- 
              Equation at cathode: Pb2+(l) + 2e- Pb(l)  


Question 3

3(a)(i)  Compound A is an alkane.

3(a)(ii)  Compound A can be used as a fuel, i.e. cooking gas/LPG.

3(a)(iii)  One necessary condition is UV light.

3(a)(iv) 

3(a)(v)  A glass rod when dipped in (concentrated) aqueous ammonia and placed in the gaseous by-product, produces dense white fumes.

3(b)(i)  A polymer is a substance with (macro) molecules built up from many smaller  repeating units, which are connected by covalent bonds: -P-P-P-P-P- or – (P)n -.

3(b)(ii)  Type of polymerization: Addition polymerization.
  
              Name of polymer: Polypropene
  
             Use of polymer: In the manufacture of bottles and containers.

3(c)(i)  The functional group present is the ester linkage (group).

3(c)(ii)  Type of polymerization: Condensation polymerization.
  
             Use of polymer: Manufacture of automobile bodies.

3(c)(iii)  Water is the by-product formed in this polymerization reaction.

Question 4


4(a)  An isotope is a species of an element having a different mass number, but  the same atomic number as another species of the    same element, due to a difference in the number of neutrons in the nuclei.

For carbon-12, number of protons = 6, and mass number, i.e. number of   (protons + neutrons) = 12 therefore number of neutrons = (12-6) = 6.

For carbon-13, number of protons = 6, and mass number, i.e. number of (protons + neutrons) = 13 therefore number of neutrons = (13-6) = 7.

So carbon-12 and   carbon-13 have the same number of protons (6) and different numbers of   neutrons (6 and 7 respectively). They are therefore isotopes of carbon.

4(b)  Radiocarbon dating uses carbon-14.
         Treatment of cancer uses cobalt-60
         Tracing of blood flow and locating of obstructions in the circulatory system   uses sodium-24.

4(c)(i)  Period number = number of occupied electron shells. Group number =   number of outer electrons. W is in period 2, group 7 and X is in period 3,   group 2.  X is a metal and W is a non-metal. The bonding between them is   ionic bonding. W forms W- and X forms X2+, so the formula of the   compound formed is XW2.

4(c)(ii)  The compound will dissolve in water. This is because it consists of positive   and negative ions whose charges result in ion-dipole attractions with water,   a polar solvent. In reality, this compound, CaF2 is one of a small number of   insoluble fluorides so the correct answer is that it is insoluble in water, but a   CSEC student normally does not know how to explain why a compound is   insoluble. I can only guess what would be done in a case where this   question was answered correctly in terms of lattice and hydration   enthalpies.

Question 5

5(a)  Ammonia is a colourless gas with a pungent odour.

5(b)(i

5(b)(ii)  2NH4Cl(s) + Ca(OH)2(s) 2NH3(g) + CaCl2(aq) + 2H2O(l)

5(b)(iii)  Ammonia gas is a weak base and will react, in a neutralization reaction ,  with sulfuric acid. Hence calcium oxide (CaO(s)) can be used as the drying   agent since CaO is basic and will not react with/neutralize the ammonia gas.

5(c)  Moist red litmus when placed in contact with ammonia gas turns blue.

5(d)  Three harmful effects of excessive nitrates in the environment are:

  1. Eutrophication of water bodies results from high nitrate content.
   
    2. Nitrates in drinking water is poisonous to humans, especially babies.

  3. Nitrosamines, which have been found in foods are carcinogenic and are thought to orignate from nitrates.

Question 6

6(a)  1. Water is a very good solvent as it dissolves both ionic and covalent compounds. Since chemical reactions occur much more readily when in solution, water is important for metabolic processes such as digestion and respiration. The ability of water to dissolve oxygen is necessary for the survival of aquatic life.

  2. Water has a high specific heat capacity and can absorb a large amount of heat without a large change in temperature. This helps living organisms, which contain approximately 70% water, to maintain a relatively constant body temperature.

6(b)(i)  Ca2+(aq) + Na2CO3(aq) CaCO3(s) + 2Na+(aq)

6(b)(ii)  Boiling can remove temporary hardness. The soluble hydrogencarbonates decompose to form the insoluble carbonates. Boiling therefore softens the water by removing the Ca2+ and Mg2+ ions that were in solution:

  M(HCO3)2(aq) MCO3(s) + H2O(l) + CO2(g)

Where M is Ca and/or Mg.

6(c)  Hard water is water that does not readily form a lather with soap.
  Soft water is water that readily forms a lather with soap.

  To distinguish between hard and soft water samples, the following method can be used. Equal volumes of both samples are added to identical test tubes and the test tubes labelled. 1 cm3 of soap solution is added to both water samples and shaken for 30 seconds. The height of lather produced by each sample is measured. The water sample giving the greater height of lather is the soft water. The hard water may either give a lower height of lather, or no lather at all depending on the degree of hardness.


Tuesday, February 9, 2016

Moles Question Solved

Question

A compound of formula M3N contains 0.673 g of nitrogen per gram of the metal M. What is the relative atomic mass of M? Identify M.

Solution

In M3N, 1 mole of nitrogen would be combined with 3 moles of M.

This means, in terms of mass, that 14 g of nitrogen would be combined with a mass of M, equal to that of 3 moles of M.

If 1 g of M was combined with 0.673 g of nitrogen.

Then 14 g of nitrogen would be combined with 20.802 g of M.

From the first line, above, this mass of M is that of 3 moles of M.

Hence molar mass of M = 6.934 g mol-1 and its relative atomic mass is 6.934.

Identity of M is lithium.

Wednesday, January 20, 2016

Chlorine Bleach




Hypochlorous acid, HClO, forms when chlorine dissolves in water. In this reaction, half of the chlorine is oxidized to hypochlorite ion and half is reduced to chloride ion in a disproportionation reaction.

Cl2(g) + 2H2O(l) H3O+(aq) + HClO(aq) + Cl(aq)

If Cl2 is dissolved in cold aqueous NaOH instead of in pure water, hypochlorite ion and chloride ion form.

Cl2(g) + 2OH(aq) ClO(aq) + Cl(aq) + H2O(l)

Under basic conditions, the equilibrium lies far to the right. The resulting alkaline solution is the “liquid bleach” used in home laundries. The bleaching action of this solution is a result of the oxidizing ability of ClO. Most dyes are colored organic compounds, and hypochlorite ion oxidizes dyes to colorless products.

When calcium hydroxide is combined with Cl2, solid Ca(ClO)2 is the product. This compound is easily handled and is the “chlorine” that is sold for swimming pool disinfection.
This information is relevant to the January 2016 P2 and can be used to answer Q 2(a).