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Showing posts with label SOME BASIC CONCEPT OF CHEMISTRY. Show all posts
Showing posts with label SOME BASIC CONCEPT OF CHEMISTRY. Show all posts

Tuesday, August 29, 2023

Average molar weight (Molecular weight): or Mean Molecular Mass

We know that for gas:

Pressure fraction = Mole fraction= Volume fraction

Pressure % = Mole % = Volume %

Consider mixture of two different gases (A) and (B) and their moles nA

and nB

Illustrated Examples:

(1) A sample of ozone gas is found to be 40% dissociated into oxygen. The average molecular mass of sample should be :








Vapour density of mixture of NO2 ​and N2O4 ​ is 34.5, then percentage abundance of NO2 ​ in mixture is:

 Answer Key: 50 %

A gaseous mixture of H2 ​and CO2 ​contains 66 mass % of CO 2 ​ The vapour density of the mixture is:

 Answer Key: VD :2.7

The vapour density of a mixture containing NO2 and N2O4 is 27.6 . Mole fraction of NO2 in the mixture is:

Answer Key: 0.2 Mole

N2O4 on decomposition gives its monomer NO2 and they are found to be in equilibrium at 300 K. if the VD of such a mixture is 41.4 the calculate % composition by (1) Moles of NO2 and N2O4 (2) By mass O2

Answer Key: (1) 20% and 80% (2) 11.1%

A compound of nitrogen and oxygen was found to contain 7: 16 by mass N and O respectively. Calculate molecular formula if vapour density is 46.

 Answer Key: ( N2O4)

A mixture of blue and green balls contains 25% by number blue balls. Mass of each blue balls is 40 gm and mass of each green balls is 20 gm calculate (1) Average mass of ball (2) % by mass

Answer Key: (1) 25 gm (2) 40 %

Air contains 20% of O2 and 80 % N2 by moles find the average molecular mass weight and % by mass of oxygen and nitrogen.

Answer Key:  Mav = 23.8 amu ,  %  by mass of Oxygen =  22.3 %  and % by mass of Nitrogen is 77.7 % 

An element X have three isotopes X ^20 ,X ^21 and X ^22 . The percentage abundance of X^20 is 90% and the average atomic mass of element is 20.18. The percentage abundance of X ^21 should be

 Answer Key: 2%


Calculate Average atomic mass of Argon the natural abundance % of isotopes of Argon is given as:

Answer Key: 39.94 amu

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 .

Answer Key: The average atomic mass of silicon is 28.1 amu


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

 Answer Key: Ag (107) 25% and Ag (109) 75%

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”

 Answer Key:  A= 10

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

 Answer Key: 1.000199 amu

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 .