# CBSE Board Exam 2020: Important MCQs (with Answers) for Class 12 Physics - Chapter 13 - Nuclei; Also useful for JEE Main, UPSEE, WBJEE & Other Engineering Entrance Exams

Check important MCQs (with Answers) for Class 12 Physics Board Exam 2020 (Chapter 13 - Nuclei). These MCQs are very useful for other competitive exams like UPSEE 2020, JEE Main 2020, WBJEE 2020 etc.  CBSE Board Exam 2020: Important MCQs (with Answers) for Class 12 Physics - Chapter 13 - Nuclei; Also useful for JEE Main, UPSEE, WBJEE & Other Engineering Entrance Exams

Check important MCQs (with solutions) for CBSE Class 12th Physics Board Exam 2020 (Chapter 13 - Nuclei). Here you will also get important links to access some important articles for the preparation of CBSE 12th board exams 2020. Students preparing for CBSE Class 12th Physics Board Exam 2020 usually ask about important MCQs (Multiple Choice Questions) & here we have provided important questions (with solution), based on Chapter 13 (Nuclei) of Class 12th Physics NCERT textbook.

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Important MCQs for CBSE Class 12 Physics Board Exam 2020 (Chapter 13 - Nuclei):

## Nuclei

Q1. The ratio of nuclear radii and nuclear densities of 26Fe56 and 92U238 is

(a) 0.671 and 1 respectively

(b) 1 and 0.671 respectively

(c) both equal to 1

(d) both equal to 0.671

Sol: (a)

Give, A1= 56, A2 = 238

Q2. There are 106 radioactive nuclei in a given radioactive element. Its half life is 20 seconds. The number of nuclei will remain left after 10 seconds are:

(a) 3 × 106

(b) 3 × 105

(c) 7 × 105

(d) 2 × 105

Sol: (c)

Given, number of initial nuclei = N0 = 106, half life = T = 20 s, time for which decay will occur = t = 10 s.

Number of half lives in t sec, n = t/T = 10/20 = 1/2

Number of nuclei after n half lives is given by:

Q3. The half life period of a radioactive material if its activity drops to (1/16)th of its initial value in 30 years is

(a) 1.5 years

(b) 3.5 years

(c) 7.5 years

(d) 15 years

Sol: (c)

If N is the final number of Nuclei left and N0 be the initial number of Nuclei then, according to the question, N = N0/16, when t = 30 years

Number of Nuclei left after n half-lives is given by

Q4. The half life off 92U238 against - decay is 4.5  109 year. The activity of 1 g sample of 92U238 is:

(a) 2.2 × 106 Bq

(b) 3.2 × 105 Bq

(c) 1.2 × 104 Bq

(d) 4.1 × 108 Bq

Sol: (c)

Given, time of half life = T = 4.5 109 years = 4.5109365246060s = 1.42  1017s

We know that,

Number of Nuclei in 1 mole of uranium or 238 g of Uranium‒238 = Avogadro’s number = 6.023 1023

Q5. The disintegration energy Q for fission of 42Mo98 into two equal fragments 21Sc49 by bombarding with a neutron is:

[Given, m (42Mo98) = 97.90541 u, (21Sc49) = 48.95002 u, mn = 1.00867 u]

(a) 341.2 MeV

(b) 944.1 MeV

(c) 5401.2 MeV

(d) None of these

Sol: (b)

The disintegration energy in fission of 42Mo98 is given by

Q = (∆m) X 931MeV = [m (42Mo98) + mn – 2m (21Sc49)] X 931 MeV

= [97.90541 + 1.00867 – 2 X 48.95002] X 931 MeV

Q = 1.01404 X 931 = 944.1 MeV

Q6. A radioactive nucleus ‘A’ undergoes a series of decays according to the following scheme :

If the mass number and atomic number of A are 180 and 72 respectively then the mass number and atomic number for A4 are:

(a) 69 and 172 respectively

(b) 115 and 71 respectively

(c) 172 and 69 respectively

(d) None of these

Sol: (c)

Emission of 2 alpha particles decreases the mass number by 8 and charge number by 4.

Emission of one β particle increases the charge number by 1 without affecting mass number.

γ emission causes no change.

Therefore, for A4: mass number = 180 ‒ 8 = 172 and atomic number 72 – 4 + 1 = 69

Q7. The number of beta particles emitted by a radioactive substance is twice the number of alpha particles emitted by it. The resulting daughter is an

(a) isobar of parent

(b) isomer of parent

(c) isotone of parent

(d) isotope of parent

Sol: (d)

With emission of one α particle decreases the charge number by 2 and emission of two β particles increases he charge number by 2. Therefore, charge number or atomic number after emission of one α particle and two β particles remains the same. Hence the resulting daughter must be an isotope of the parent involving decrease in mass number by 4.

Q8. In a sample off radioactive substance, what percentage decays in one mean life time?

(a) 70 %

(b) 63.2 %

(c) 45.8 %

(d) 23%

Sol: (b)

Given, t = τ = 1/λ,

⇒ N/N0 = e-λt = e-1 = 1/e = 1/2.72 = 0.36

Fraction decayed %

Q9. The binding energy per nucleon for the parent nucleus is E1 and that for the daughter nuclei is E2. Then

(a) E1 > E2

(b) E2 > E2

(c) E1 = 2E2

(d) E2 = 2E1

Sol: (b)

When a heavy nucleus of higher mass number (less stable) splits into two lighter nuclei the daughter nucleus is of less mass number and becomes more stable, having more binding energy per nucleon. Therefore, E2 > E1

Q10. Half-lives of two radioactive substances A and B are respectively 20 minutes and 40 minutes. Initially the samples of A and B have equal number of nuclei. Find the ration of remaining number of A and B nuclei after 80 minutes.

(a) 1 : 1

(b) 4 : 1

(c) 1 : 4

(d) 1 : 16

Sol: (c)

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