# List of Posts written during Apr 2021

This is a list of of posts written during the month Apr 2021
(Rev. 28-Jun-2024)

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### List of Posts

This is the complete list of categories of posts written during Apr 2021. They have been ordered by the publish date, with the most recent first.

1. Published: 01-Apr-2021
A calorie is a unit of heat (energy in transit) and it equals about 4.2 J where 1J = 1 kg m2 s-2. Suppose we employ a system of units in which the unit of mass equals α kg, the unit of length equals β m, the unit of time is γ s. Show that a calorie has a magnitude 4.2 α-1 β-2 γ2 in terms of the new units.

2. Published: 03-Apr-2021
A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in terms of the new unit if light takes 8 min and 20 s to cover this distance?

3. Published: 03-Apr-2021
Which of the following is the most precise device for measuring length : (a) a vernier callipers with 20 divisions on the sliding scale (b) a screw gauge of pitch 1 mm and 100 divisions on the circular scale (c) an optical instrument that can measure length to within a wavelength of light?

4. Published: 04-Apr-2021
The mean diameter of a thin brass rod is to be measured by vernier callipers. Why is a set of 100 measurements of the diameter expected to yield a more reliable estimate than a set of 5 measurements only? . . . and 2 more questions

5. Published: 05-Apr-2021
The photograph of a house occupies an area of 1.75 cm2 on a 35 mm slide. The slide is projected on to a screen, and the area of the house on the screen is 1.55 m2. What is the linear magnification of the projector-screen arrangement.

6. Published: 07-Apr-2021
State the number of significant figures in the following : (a) 0.007 m2 (b) 2.64 × 1024 kg (c) 0.2370 g cm-3 (d) 6.320 J (e) 6.032 N m-2 (f) 0.0006032 m2

7. Published: 07-Apr-2021
The length, breadth and thickness of a rectangular sheet of metal are 4.234 m, 1.005 m, and 2.01 cm respectively. Give the area and volume of the sheet to correct significant figures.

8. Published: 07-Apr-2021
The mass of a box measured by a grocer's balance is 2.30 kg. Two gold pieces of masses 20.15 g and 20.17 g are added to the box. What is (a) the total mass of the box, (b) the difference in the masses of the pieces to correct significant figures.

9. Published: 08-Apr-2021
A physical quantity P is related to four observables a, b, c and d as follows : P= a^3b^2/√c d The percentage errors of measurement in a, b, c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P ? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result ?

10. Published: 09-Apr-2021
A book with many printing errors contains four different formulas for the displacement y of a particle undergoing a certain periodic motion (a) y = a sin 2π (t/T) (b) y = a sin vt (c) y = (a/T) sin (t/a) (a = maximum displacement of the particle, v = speed of the particle. T = time-period of motion). Rule out the wrong formulas on dimensional grounds.

11. Published: 10-Apr-2021
A famous relation in physics relates 'moving mass' m to the 'rest mass' mo of a particle in terms of its speed v and the speed of light, c. (This relation first arose as a consequence of special relativity due to Albert Einstein). A boy recalls the relation almost correctly but forgets where to put the constant c. He writes : m=m0/√(1-v2). Guess where to put the missing c.

12. Published: 12-Apr-2021
One mole of an ideal gas at standard temperature and pressure occupies 22.4 L (molar volume). What is the ratio of molar volume to the atomic volume of a mole of hydrogen ? (Take the size of hydrogen molecule to be about 1 Å). Why is this ratio so large ?

13. Published: 12-Apr-2021
Explain this common observation clearly : If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train's motion, but the distant objects (hill tops, the Moon, the stars etc.) seem to be stationary. (In fact, since you are aware that you are moving, these distant objects seem to move with you).

14. Published: 13-Apr-2021
A parsec is a convenient unit of length on the astronomical scale. It is the distance of an object that will show a parallax of 1" (second of arc) from opposite ends of a baseline equal to the distance from the Earth to the Sun. How much is a parsec in terms of metres ?

15. Published: 14-Apr-2021
The nearest star to our solar system is 4.29 light years away. How much is this distance in terms of parsecs? How much parallax would this star (named Alpha Centauri) show when viewed from two locations of the Earth six months apart in its orbit around the Sun ?

16. Published: 15-Apr-2021
The Sun is a hot plasma (ionized matter) with its inner core at a temperature exceeding 10^7 K, and its outer surface at a temperature of about 6000 K. At these high temperatures, no substance remains in a solid or liquid phase. In what range do you expect the mass density of the Sun to be, in the range of densities of solids and liquids or gases? Check if your guess is correct from the following data : mass of the Sun = 2.0 × 10^30 kg, radius of the Sun = 7.0 × 10^8 m.

17. Published: 15-Apr-2021
When the planet Jupiter is at a distance of 824.7 million kilometers from the Earth, its angular diameter is measured to be 35.72" of arc. Calculate the diameter of Jupiter.

18. Published: 15-Apr-2021
A man walking briskly in rain with speed v must slant his umbrella forward making an angle θ with the vertical. A student derives the following relation between θ and v : tan θ = v and checks that the relation has a correct limit: as v →0, θ →0, as expected. (We are assuming there is no strong wind and that the rain falls vertically for a stationary man). Do you think this relation can be correct ? If not, guess the correct relation.

19. Published: 16-Apr-2021
It is claimed that two caesium clocks, if allowed to run for 100 years, free from any disturbance, may differ by only about 0.02 s. What does this imply for the accuracy of the standard cesium clock in measuring a time-interval of 1 s ?

20. Published: 16-Apr-2021
Estimate the average mass density of a sodium atom assuming its size to be about 2.5 Å. (Use the known values of Avogadro's number and the atomic mass of sodium). Compare it with the mass density of sodium in its crystalline phase : 970 kg m-3. Are the two densities of the same order of magnitude ? If so, why ?

21. Published: 17-Apr-2021
The unit of length convenient on the nuclear scale is a fermi : 1 f = 10^-15 m. Nuclear sizes obey roughly the following empirical relation : r = r0 A^1/3 where r is the radius of the nucleus, A its mass number, and ro is a constant equal to about, 1.2 f. Show that the rule implies that nuclear mass density is nearly constant for different nuclei. Estimate the mass density of sodium nucleus. Compare it with the average mass density of a sodium atom obtained in Exercise. 2.27.

22. Published: 17-Apr-2021
A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances. The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon's surface. How much is the radius of the lunar orbit around the Earth ?

23. Published: 18-Apr-2021
A SONAR (sound navigation and ranging) uses ultrasonic waves to detect and locate objects under water. In a submarine equipped with a SONAR, the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be 77.0 s. What is the distance of the enemy submarine? (Speed of sound in water = 1450 m s-1).

24. Published: 18-Apr-2021
It is a well known fact that during a total solar eclipse the disk of the moon almost completely covers the disk of the Sun. From this fact and from the information you can gather from examples 2.3 and 2.4, determine the approximate diameter of the moon.

25. Published: 18-Apr-2021
Calculate the molar mass of the following: (i) H2O (ii) CO2 (iii) CH4

26. Published: 18-Apr-2021
Calculate the mass per cent of different elements present in sodium sulphate (Na2SO4).

27. Published: 18-Apr-2021
Determine the empirical formula of an oxide of iron, which has 69.9% iron and 30.1% dioxygen by mass.

28. Published: 21-Apr-2021
Calculate the amount of carbon dioxide that could be produced when (i) 1 mole of carbon is burnt in air. (ii) 1 mole of carbon is burnt in 16 g of dioxygen. (iii) 2 moles of carbon are burnt in 16 g of dioxygen.

29. Published: 21-Apr-2021
Calculate the mass of sodium acetate (CH3COONa) required to make 500 mL of 0.375 molar aqueous solution. Molar mass of sodium acetate is 82.0245 g mol-1.

30. Published: 22-Apr-2021
Calculate the concentration of nitric acid in moles per litre in a sample which has a density, 1.41 g mL-1 and the mass per cent of nitric acid in it being 69%.

31. Published: 22-Apr-2021
How much copper can be obtained from 100 g of copper sulphate (CuSO4)?

32. Published: 23-Apr-2021
Calculate the atomic mass (average) of chlorine using the following data: % natural abundance of 35Cl is 75.77% and molar mass is 34.9689 g/mol. The % natural abundance of 37Cl is 24.23% and its molar mass is 36.9659 g/mol.

33. Published: 24-Apr-2021
In three moles of ethane (C2H6), calculate the following: (i) Number of moles of carbon atoms (ii) Number of moles of hydrogen atoms (iii) Number of molecules of ethane.

34. Published: 25-Apr-2021
What is the concentration of sugar (C12H22O11) in mol L-1 if its 20 g are dissolved in enough water to make a final volume up to 2L?

35. Published: 28-Apr-2021
If the density of methanol is 0.793 kg L-1, what is its volume needed for making 2.5 L of its 0.25 M solution?

36. Published: 29-Apr-2021
Pressure is determined as force per unit area of the surface. The SI unit of pressure, pascal is as shown below: 1Pa = 1N m-2. If mass of air at sea level is 1034 g cm-2, calculate the pressure in pascal.

37. Published: 30-Apr-2021
A sample of drinking water was found to be severely contaminated with chloroform, CHCl3, supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass). (i) Express this in per cent by mass. (ii) Determine the molality of chloroform in the water sample.

38. Published: 30-Apr-2021
In a reaction A + B2 → AB2 Identify the limiting reagent, if any, in the following reaction mixtures. (i) 300 atoms of A + 200 molecules of B (ii) 2 mol A + 3 mol B (iii) 100 atoms of A + 100 molecules of B (iv) 5 mol A + 2.5 mol B (v) 2.5 mol A + 5 mol B