Some Basic Concepts of Chemistry - Online Test

Calculations:

Mass of 1mole of  O2

  =  the mass of 6.022 x 1023  molecules  of  O2     ' ( as per Avogadro's law )

= molar mass of O2  in grams ( ie. gram molar mss )

=  32 .0  g  

 Since , the number of molecules contained in one mole of a substance

 =  Avogadro's number  (N)

 =   6.022 x 1023  molecules ,

and  , gram molar mass of water

= 18 g  

∴$\therefore$18g H2O =   1mol water

∴$\therefore$ one mol of water would contain

= 6.022 x 1023 molecules of water 

Chemical reactions rarely  occur completely when exactly the right amounts of reactants are taken to react together  to yield products as per its stoichiometry .One of the  reactants  will be used up  ,before another runs out.Thus , the reactant which  is consumed first is known as limiting reagent 

For  the given reaction ,

2 N2 H4 (l)      +     N2 O4  (l)  ------>   3 N2 (g)     +  4 H2 O (l)

as per its stoichiometry , if  only the right and exact amount of  N2 O4 is taken , the reaction may not go to completion .As such it would be consumed first & is , therefore , the  limiting reagent. 

Step  1  

Number of  moles of H2  in 67.2 L  of  H2 

=67.222.4

=3 moles

Step 2 /  

Number of molecules in  1 moles of  H2 

=6.02 x 1023  molecules of H2

 Since  H2  is a diatomic gas  the number of atoms in 1 mole of  H2 

=  ( 2   x  6.02 x  1023 )     

  = 12.04 x 1023  atoms of H atoms  

(since ,  one molecule of H2 contains 2 atoms ).

∴$\therefore$ Number of atoms in 3 moles of H2 

=  ( 3   x  12.04  x  1023  ) 

=3.612x 1024  atoms of H.

Chemistry does not deal in explaining superconductivity.

*In superconducting materials the charecteristics of superconductivity appear when the temperature is lowered below a critical temperature.

*The onset of superconductivity is accompanied  by abrupt changes in physical properties which are more related to phase transitions of the material .

These aspects of  studies in properties of materials  are better related to studies in the fields of Physics , eventhough  principles involved in Chemistry & Physics go hand to hand.  

Since,  density

= mass / volume ;

 and  SI unit of mass is kg and that of volume is m3

∴$\therefore$ using  these  SI units the unit of density is derived through dimensional analysis :

density  =   mass /  volume

 =   kg  /  m3 

or, =  kg m-3   

Molecular formula is the exact no. of atoms present in a molecule of a compound.

Molecular formula of a compound is related with its empirical formula  as ,

Mollecular formula  =  ( Empirical formula ) n 

where n represents a positive integer .

Since ,  1L

= 1000mL  

   or , 1mL

 =11000L.

∴$\therefore$  0.05mL

=  11000X0.05L  

=  0.00005 L .

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g of the 12C isotope .

A mole of a substance always contains the same number of entities , no  matter what the substance may be. In order to determine this   number precisely , the mass of carbon -12 atom was determined by mass spectrometer  and found to be equal to 1.992648 x 10-23 g. Knowing that one mole of carbon weighs 12 g . the number of atoms in it is equal to :

6.0221367 x 1023