46. D
47. A
48. D
49. B
50. D
51. D
52. b) 7 x 105
53. 4.01 x 10 4 Bq
54. 3.8 x 10-12 kg
55. 3.75 x 107 Bq
57. d) 1/132
46. Find the activity after 2 years. Activity and time is inversely proportional. Hence A1T1 = A2T2.
A2 = 0.707 A1
Hence T2= 10/0.707 = 14 min
47. Actual count rate at 10 days is 60-20 = 40. At 30 days (after 20 days) is 30-20 = 10 . Hence 20 days is 2 half life (Count rate fall from 40 to 20 to 10). Hence one half life is 10 days.
48. Half the y axis readings should give equal time duration (half life) on the x axis. The best graph is D.
49. A is proportional to N . N/Na = M/Mr. Subs the correct symbols in question to include mass .
50. Initial count rate is 80 s-1 after deducting background count rate. After one half life the count rate is 40 +10 = 50 s-1 , expected count rate including the back ground count rate.
51.Find each activity separately and the total activity is the sum of the two activity.
52. Unit for decay constant is s-1.
53. Find the initial activity.
54. Find the number of atoms that would yield the activity first . Then use the formula m/M = N/Na to find the mass m.
55. Find the rate of charge flow = I/e. These charges are both positive and negative ions so the rate of ion pairs is I/2e. Each alpha produces 2.0 x 105 ion pairs . Hence activity is No of ion pairs per sec divide by number of ion pairs formed by one alpha.
Note : I sometimes forget to divide by 2 to change number of ions to ion pairs.
57. 5 half life is (1/2)5 . Hence fraction is 1/132.
