calculate An ideal gas (the system) is contained in a flexible balloon at a pres

Question

calculate An ideal gas (the system) is contained in a flexible balloon at a pressure of 1 atm and is initially at a temperature of 20 degree C. The surrounding air is at the same pressure, but its temperature is 25 degree C. When the system has equilibrated with its surroundings, both systems and surroundings are at 25 degree C and 1 atm. In changing from the initial to the final state, which one of the following relationships regarding the system is correct? Calculate the pH of a 0.30 M solution of aqueous ammonium bromide. The value of K_b for NH_3 us 1.8 Times A 0.505 g sample of a monoprotic base (MM = 45.09 g/mol) was dissolved in water to produce 100.0mL of solution with a pH = 11.84. What is the ionization constant of this base? The highly exothermic thermite reaction, in which aluminium reduces iron(III) oxide to elemental iron, has been used by railroad repair crews to weld rails together. What mass of iron is formed when 725 kJ of hear are released?

Explanation / Answer

26.

In changing from the initial to the final state, as there is no change in the pressure but there is in the temperature we can say that there is change in the INTERNAL ENERGY, it goes up as the temperature goes up, so delta E has to be mayor to zero, there is no option, so what happens with volume when temperature goes up to at constant pressure? Volume is increased: So work is done by the system over the surroundings, so w goes up, OPTION D

27.

Ammonium bromide is soluble in water. The Bromide ion does not hydrolyze, but the ammonium ion does.

NH4Br(aq) --> NH4+ + Cl-
NH4+ + H2O <==> NH3 + H3O+ ....... Ka = ???
Ka = Kw / Kb = 1.00x10-14 / 1.8x10-5 = 5.56x10-10

Ka = [NH3] [H3O+] / [NH4+]
5.56x10-10 = x² / 0.30
x = 1.29x10-5

H3O+ = 1.29x10-5 M
pH = -log [H3O+] = 4.88... OPTION A

28. First the molarity of our solution:

Molarity: Mass in g/(Molecular weight x Volume in Liters)

M= 0.505 /(45.09x0.1)= 0.112

Our reaction goes like this:

       Monoprotic base (MB) ------------- OH   + Ionized Base(IB)

I          0.112                                    0          0

C         -x                                        +x        +x

E         0.112-x                               x              x

as pH is 11.84.... pOH= 14-pH= 2.16.... [OH-]= 10-pOH=6.91x10-3 =x

Kb= IBxOH/MB

kb= x2/(0.112-x)= (6.91x10-3)2/(0.112-6.91x10-3)= 4.55x10-4...Rounding up is OPTION C

29.

When 850 kJ are released 2 Fe= 2 x 55.8= 111.6g are formed

850 kJ ------------- 111.6 g

725 kJ --------------- x          x= 725x111.6/850= 95.18 g, OPTION C

30. Combustion of Ethanol

C2H5OH + 3O2 ------------ 2CO2 + 3H2O

Hrxn= Summ of Hf (products) - summ of Hf (reactants)

standard enthalpy of formation of O2 is 0(for elementary substances in the standard state the standard enthalpy of formation is taken as zero)

Hrxn= [2 X -393.5 + 3 X -241.8] - [-277.7 + 3 X 0]

Hrxn= -1234.7 kJ/mol OPTION A

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