Carbon occurs in nature as a mixture of carbon 12 and carbon 13. The average atomic mass of carbon is 12.011. What is the percentage abundance of carbon 12 in nature?

SOLUTION:

Calculate the molar mass of naturally occurring argon. Using the date given in the following table to ..

 

SOLUTION: Molar mass of Ar

= 35.96755 × 0.071 + 37.96272 × 0.163 + 39.96924 × 0.766

= 39.352 g mol−1 

Average Atomic Mass/weight or Relative Atomic Mass:

Elements are found in different isotopic forms (atoms of same elements having different atomic mass), so the atomic mass of any element is the average of all the isotopic mass within a given sample. 

(A) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’ in the ratio   X: Y, then

or

(B) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’  and their abundant  percentage (%)  are  X % and Y % then

Since the atomic mass is a ratio, it has no units and is expressed in amu.

Illustrative Examples:

(1) Calculate the average atomic mass of silicon if relative abundance is 92.23 ,isotope Si^28, 4.77 % isotope = Si^29 and 3% isotope = Si^39 .

(2) Silver contains two isotopes Ag 107 and Ag109 and average atomic weight of silver is 108.5 find percentage abundant of isotopes of silver.

(3) Boron have two isotopes B11 and BA and their relative abundant % are 80% and 20% respectively and average atomic weight of Boron is 10.8 find the value of “A”

(4) Natural hydrogen gas is a mixture of 1H and 2H in the ratio of 5000:1. Calculate the atomic mass of the hydrogen.

(5) Carbon occurs in nature as a mixture of carbon 12 and carbon 13. The average atomic mass of carbon is 12.011. What is the percentage abundance of carbon 12 in nature?

(6) Calculate the molar mass of naturally occurring argon. Using the date given in the following table to ..

Relation between Kp and Kc and Law of Mass of action: Equilibrium constants:

LAW OF MASS ACTION: Law of mass action is applicable for only reversible chemical reactions and it is an imperial law.

The law state that “ At a fixed temperature the rate of a chemical reaction is directly proportional to the product of reactive mass of reactants raised to the their respective Stoichiometric coefficients ” The law of mass action is by Guldberg and Waage.

(1) Equilibrium Constants in term of concentration ( K_C):

At the constant temperature, let us consider the following reversible reaction

According to law of mass action - The rate of forward reaction

The rate of reverse reaction-

Where K_f and K_b  is the rate constant of the forward reaction and backward reaction respectively

We know at equilibrium, the two rates of forward as well as backward are equal. ie

Rate of reaction = Rate of forward reaction – Rate of backward reaction = 0

Kc=Kf/Kb

Unit of Kc= (Conc)ng

(2) Equilibrium Constants in term of Pressure( Kp):

Consider the general chemical  reaction taking place at constant temperature.

From law of mass action- rate forward reaction is directly proportional  to product of active mass of reactants and rate backward reaction is directly proportional  to product of active mass of products.

For an ideal gas PV=nRT

Where 

P= Pressure in atm

V=Volume in liters

n=Number of gaseous moles

R=Gas constant  

  =  0.0821 L atm/mol/K or 1/12 L atm /mole/K

T=Temperature in kelvin

                                      = total number of moles of gaseous products -total number of moles of gaseous reactants

Illustrated examples:

(1) The chemical equilibrium constant Kc for the decomposition of PCl5 is 0.0625 mole L^-1 at 300^C. What will be the value of Kp is ?

(2) The value of Kc for the reaction (formation of ammonia by haber process) is 0.50 at 400º C.What will be the value of Kp at 400ºC when concentration are expressed in mole litre.1 and pressure in atmosphere ?

(3)For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) The Kp is 4.3 × 10^-4 at 375°C. Calculate Kc for the reaction. ?

(4) Methanol (CH3OH) is manufactured industrially by the reaction CO(g) + 2H2(g) ⇌ CH3OH(g) The equilibrium constant (Kc) for the reaction is 10.5 at 220°C. What is the value of Kp at this temperature ?

(5) At 700 K, the equilibrium constant Kp, for the reaction, dissociation of Sulphur trioxide to Sulphur dioxide is 1.8 × 10^3 kPa What is the numerical value of Kc for this reaction at the same temperature

The chemical equilibrium constant Kc for the decomposition of PCl5 is 0.0625 mole L^-1 at 300^C. What will be the value of Kp is ?