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1- Molecular mass.

posted Mar 3, 2016, 8:25 PM by Upali Salpadoru   [ updated Jun 5, 2017, 1:31 PM ]


 How do we give the mass of atoms? 


They are so small and we cannot use the normal units.  

We can use the lightest atom to get the mass of atoms. For example a carbon atoms weighs as much as 12 Hydrogen atoms. So we can say the mass number of H is 1 and C is 12.  This is approximately correct.

But there is a hitch here. All hydrogen atoms do not have the same mass. There are the heavy Hydrogen atoms, which weigh as twice as much as the normal Hydrogen. 

Such similar atoms having different masses are called isotopes. Almost all elements have isotopes.

Fig,1 A diagram of a Carbon Atom.

So the chemists have defined the Atomic mass unit with respect to a Carbon isotope. Namely Carbon-12.

Carbon -12 isotope is taken as exactly 12 amu .(Atomic mass units)

According to this one Atomic mass unit  is 1/12th  the mass of Carbon 12 isotope.

According to this system H becomes = 1.008.amu.

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Imagine a scale balance capable of weighing atoms.  If you have an atom of Berilium on one side of a scale balance How many hydrogen atoms would be necessary to balance it?

You must refer a Periodic Table to get the atomic mass of elements.



Fig2. Comparing the mass of atoms.

 Relative Atomic Mass. (Atomic Weight).

The Periodic Table does not give a unit for the Atomic Mass. This is just a number showing how many times an average atom of an element weighs as much as a Carbon-12 isotope taken as 12 units.

What is given in the Periodic Table is the RAM.

Molecular Mass.


 Using the formula  of the water molecule, we can get the mass.You have only to add the atomic masses of the individual atoms.






Examples:

 H2O NaCl Na2CO3 H3PO4
 2+16=18 23+35.5
= 58.5
 46+12+48
=106
 3+31+68
=102

 






Part--1

1.0 MCQ.  Define the following:-

1.   Atomic Mass Unit. (u or Dalton)

  1. The mass of 1/12th part of the Carbon --12 isotope.

  2. The mass of 1/12th part of the Carbon atom.

  3. The mass of 1/12th part of an average Carbon atom.

  4. The mass of 1 Hydrogen atom.


2.  Atomic mass.

  1. The sum of the nucleons and electrons in  an isotope.

  2. Mass of a particular isotope expressed in Atomic Mass Units.

  3. Number of times an atom is heavy as the hydrogen atom.

  4. The average mass of an atom of the element in atomic mass units.


3. Atomic number:-

  1. The number of protons in the nucleus of an atom,

  2. The number of electrons in the atom,

  3. A number obtained by arranging the elements according to the atomic weights.

  4. The electrical charge of an atom.

4.   Mass number:-
a. The total number of nucleons in an isotope.
b. The weight of an atom compared to the weight of a carbon isotope
c. The absolute mass of an atom.
d. The total number of protons in the nucleus
5. Relative Atomic mass:-
a. The number of protons and electrons of an atom.
b. Number of times an atom is heavy as the hydrogen atom.
b. The average mass of an atom relative to 1/12th the mass of the Carbon --12 isotope.
d. The mass of an isotope relative to 1/12th the mass of a carbon atom.
Highlight for answers.
Marks 4x5= 20.
1 a 2 b 3 a 4 d 5 c
Part--2.

1.0 Fill in the cages in this table according to the diagram of an atom given.
17Cl37


 Protons Electrons Nucleons Atomic no. Mass no.
     
Marks: 2x5 =10


2.0 This is an imaginary scale to compare the weights of isotopes and molecules.

Give the number of particles necessary to balance the two sides.
 Laft Right   Left Right
  6C12 ...........2 12Mg24-. ...........1   6C14-- a. 1H1 b.
 16S32 c. 8O16-- d.  26Fe56 e. 14Si28 f.
 7N14 g. 2He4 h.  24Cr52 i. 8O16......... .j.
2x10=20.

3.0 Give the Molecular Mass for the following:-
 O3 HCl NO2 H2SO4 Ca(OH)2 NH4Cl C2H6 H3PO4 Al2(SO4)3 CuSO4.5H2O
          .
2x10=20
4. If the number of oxygen atoms in 16 g. of oxygen is ‘x’  find the number of atoms in 2 g. of oxygen.
5. Pure chlorine has two isotopes. Cl-35  and Cl- 37. If Cl-35 makes 75.5% while the balance is Cl-37. What is the average Atomic mass of chlorine?
             6. Air consists mainly of four parts of  Nitrogen N2 to  one of Oxygen O2. What is the average mass of an air molecule?
             7 If a kilogram of Hydrogen has y atoms , what mass of Carbon will have the same number of Carbon atoms?
8. Four grams  of a particular perfume was released in a room having a volume of 50,000 litres. John came into the room felt the smell in his first breath. If the breath had a capacity of 10 ml and assuming that at least one particle of scent must be there to feel the smell, calculate the mass of a particle.        

=30

.

   Part 2 Answers

1. 

 Protons Electrons Nucleons Atomic no. Mass no.
 17 17 37 17 3

10

 2.

 Laft Right   Left Right
  6C12 ...........2 12Mg24-. ...........1   6C14-- a. 1 1H1 b. 14
 16S32 c. 1 8O16-- d. 2  26Fe56 e. 1 14Si28 f. 2
 7N14 g. 2 2He4 h. 7  24Cr52 i. 4 8O16......... .j. 13

       20

3.

 O3 HCl NO2 H2SO4 Ca(OH)2 NH4Cl C2H6 H3PO4 Al2(SO4)3 CuSO4.5H2O
 48 36.5 46 98 74 53.5 30 98  162 250
2x10=20

                                               

.4. If the number of oxygen atoms in 16 g. of oxygen is ‘x’  find the number of atoms in 2 g. of oxygen.

      16 g of O2  contains  x atoms.                                                                                                         1g    of O2  contains  x/16 atoms.                                                                                                     2g    of O2  contains  x' x2÷16  atoms..........  =x/8 atoms.

     5. Pure chlorine has two isotopes. Cl-35  and Cl- 37. If Cl-35 makes 75.5% while the balance is
         Cl-37. What is the average Atomic mass of chlorine?
        35x75.5  =2642.5
        37x 24.5 =906.5          Total =  35.49  mass of 100 atoms.
         Relative Molecular mass of Chlorine =  35.5.
       
      6. Air consists mainly of four parts of  Nitrogen Nto  one of Oxygen O2. What is the average mass          of an air molecule?
Answer:-
         Mass of 4 Nitrogen molecules= 4 x 28 =   112
         Mass of an Oxygen molecule =  32
         Mass of 5 air molecules=  112 = 32  =  144
         Mass of 1 air molecules= 144/5  =28.8
       
30



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