7.7.3 Equilibria

QUESTIONS

Section A

For each of the questions only one of the lettered responses (A – D) is correct.

 

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

 

1.            Cyclohexane and methylbenzene form an ideal mixture obeying Raoult’s Law.  Use the following data to calculate the vapour pressure in Pa at 20oC of a mixture containing 42 g of cyclohexane and 23 g of methylbenzene.

                                                Formula            VP at 20oC

          Cyclohexane                    C6H12                10241 Pa

          Methylbenzene                C7H8                 2726 Pa

 

          (Relative atomic masses: H=1; C=12)

 

          A  5919

          B  7736

          C  12967

          D  492820

 

2.     A gaseous mixture of 6.0 g of propan-1-ol and 7.4 g of butan-2-ol has a total pressure of 110 kPa at a particular temperature.  What is the partial pressure of the butan-2-ol in the mixture at this temperature?

 

A    5.5 kPa

          B   11.0 kPa

          C   49.2 kPa

          $D   55.0 kPa

 

3               The partition coefficient of a solid X between ether and water is 4. What mass of X will be extracted when 10 cm3 of ether is shaken with 100 cm3 of water containing 5.00 g of X?

 

A  1.25 g

B  1.43 g

C  3.57 g

D  4.00 g

 

4          The phase diagram for mixtures of tin and lead is shown below. Which one of the compositions  A, B, C or D will melt over a range of temperatures?

 

                                                                                                                                327

         temperature        

                       /oC

                       

                     232                                                                                                                                                                                                    

                                                                                                                               

 


                             A                      B                 $C                                                    D   

                        100% Sn                       percentage composition             100% Pb

 

Section B

 

1               In order to counter the effects of accidental fires, many stores have fitted sprinkler devices. These are fitted with plugs, made of a mixture of two metals, which withstand the pressure of water behind them. When the temperature rises above a particular value, water is released as a spray. The diagram below is a phase diagram for two such metals A and B.

2                

 


                       242

 

          temperature                                                                                                    185

                        /oC

                                                                                                                               

                                                                                                                                80

 


                        100% A                                                        72% B                   100% B

                                                            percentage composition

(a) (i)   A fire starts in a store and a plug with composition 60%B and 40% A is exposed to  temperatures which rise from 20oC to 300oC over a short period of time. Explain, using a graph of temperature against time, what happens to the metal;ic mixture in the plug as the temperature rises.

 

 

 

 

 


            temperature oC

 

 

 


                                                                        time

 

                                                                                                                                                                                              

                                                                                                                                    

 

                                                                                                                                    

 

                                                                                                                                    

 

                                                                                                                                    

                                                                                                                                    [4]

 

(ii)          How would the graph differ if the plug had the composition of 72% B?

 

 

 

 

 


            temperature oC

 

 

 


                                                                        time

 

                                                                                                                                                                                              

                                                                                                                                    

 

                                                                                                                                    

 

                                                                                                                                    

 

                                                                                                                                    

                                                                                                                                    [3]

 

(iii)        What is the name given to the mixture containing 72% B?

                                                                                                                                    

                                                                                                                                    [1]

 

(b)          If the relative atomic masses of A and B are 127 and 181 respectively, calculate the mole fraction of A in this mixture.

 

 

 

 

 

 

 

 

 

 

 

                                                                                                                                    

                                                                                                                                    [2]

                                                                                                                        CHM3J8