Q. You determine that 187 J of heat is required to raise the temperature of 93.45 g of silver from 18.5 oC to 27.0 oC. What is the specific heat capacity of silver?

Sol: The heat change with change in temperature can be related as q ms T= ∆

where q is heat change m is mass of matter, s is specific heat capacity, and

∆T is change in temperature. On solving.

1 1187 0.235493.45 8.5

qs JK gm T

− −= = =∆ ×

Q You add 100.0 g of water at 60 oC to 100 g of ice at 0.00 oC. Some of the ice melts and cools the water to 0.00 oC. When the ice and water mixture has come to a uniform temperature of 0.00 oC, how much ice has melted?

Sol: heat absorbed by ice during melting = heat lost by water on cooling There fore I f wm L m s T= ∆ where Im is mass of ice melted = ? fL is latent heat of fusion of ice = 6.008 kJ/mol = 333.778 J/g wm is mass of water =100g s is specific heat of water = 4.18 J/g ∆T is change in temperature = 60 On solving

100 4.18 60 75.14333.778

wI

f

m s Tm gL∆ × ×

= = =

Q: Suppose that only two 45g ice cubes had been added to your glass containing 25 10× ml of tea. When thermal equilibrium is reached, all of the ice have melted, and the temperature of the mixture will be some where between 20.0 oC to 0.0 oC. Calculate the final temperature of the beverage. ( Note: The 90 g of water formed when ice melts must be warmed to final temperature)

Sol: Heat absorbed by ice cubes during melting + Heat absorbed in increasing temperature of water = Heat lost in cooling of tea

Therefore 90 333.778 90 4.18 ( 0) 500 4.18 ( 20)I f w w t tm L m s T m s T t t+ ∆ = − ∆ = × + × × − = − × × − = 90 333.778 376.2 2090 41800t t× + = − +

= 41800 30040.02 2466.2t− = = 11759.98 4.7682466.2

t = =

Q: Cobalt- 60 is a radioactive isotope used in medicine for treatment of certain cancers. It

produces β particles and γ rays, the latter is having energies of 1.173 and 1.332 MeV ( 61 10MeV = electron volts and 41 9.6485 10 /eV J mol= × ) What are the wavelength and

frequency of γ ray photon with an energy of 1.173MeV? Sol: Energy of any radiation can be obtained by using planks equation i.e. E hυ= energy of photon

is 6 4

1323

1.173 10 9.6485 10 1.8794 106.022 10

E J−× × ×= = ×

×

On solving13

21 2034

1.8794 10 0.2837 10 2.837 106.625 10

Eh

υ−

−

×= = = × = ×

×Hertz

The wave length is calculated as 8

1220

3 10 1.0574 10 1052.837 10

oc mλυ

−×= = = × = Α

×

Q: When Sojourner spacecraft landed on Mars in 1997, the planet is approximately 77.8 10× Km from earth. How long did it take for the television picture signal to reach Earth from Mars?

Sol: All electromagnetic waves travel at the same speed of 83 10 /m s× so from a distance of 77.8 10× km is equal to 107.8 10× m. Time required for traveling of signal = distance/speed that

is 10

28

7.8 10 2.6 103 10

t ×= = ×

× sec.

Q: Why is the radius of Li+ , so much smaller that the radius of lithium? Why is the radius of F − so much larger than the radius of F?

Sol: Radius of an atom is defined as the distance from center of nucleus to the point where significant probability of finding electron exists. Radius of lithium ion is smaller then the radius of lithium because lithium is having electronic configuration 2 11 2s s which on ionization becomes 2 01 2s s thus last electron of lithium ion is in first shell and not second as in lithium atom.

Radius of fluoride ion is greater then fluorine atom because direct measurement of radius is not possible so radius is measured by alternate means for fluorine the radius is covalent radius that is mean of internuclear distance of covalent molecule. While radius of fluoride ion is its ionic radius that is mean of internuclear distance of its compounds. Further addition of an extra electron increases repulsion among electrons of the valence shell.

Q: Compare the configurations below with two electrons located in p orbitals. Which would be the least stable? Explain your answers.

a) b) c) d) Sol: The lowest energy is represented by option d as here electrons are unpaired and having

parallel spin. Spin of electron is giving it a magnetic character, parallel spin means same magnetic orientation. In (A) electron with parallel spin are in same orbital thus huge repulsions. In (B) electrons have opposite spin thus attract each other, but in this configuration multiplicity is limited, whereas electron tends to attain maximum multiplicity. In (C) spin is opposite as well as unpaired but reversal of spin is energy consuming. So the best configuration is option (D).

Q: Write electron configurations to show the first two ionization process for potassium. Explain

why second ionization energy is much greater then the first. Sol: The electronic configuration of potassium atom is 2 2 6 2 6 11 2 2 3 3 4s s p s p s . While that of 2 2 6 2 61 2 2 3 3K s s p s p+ = and that of 2 2 2 6 2 51 2 2 3 3K s s p s p+ = Second ionization energy of potassium is much higher then first because by using first

ionization energy potassium is acquiring configuration of a noble gas which is relatively stable system, but by using 2nd ionization energy ion loses noble gas configuration hence become unstable. Half filled and fully filled configuration are relatively more stable then other configurations because in these configuration distribution of electrons is symmetrical around the nucleus