Here in question no. 2.28  how is the mass no. taken to be 3 in the expression 
 
Mass of nucleus= A/ Avogadro no. By explaining this expression?
 
 
 
 
 
 
 
 
 

Asked by Varsneya Srinivas | 19th Jul, 2016, 11:44: AM

Expert Answer:

Mass of a nucleus is the ratio of mass number to the Avogadro's number. 
Now, this expression comes about because if we consider one nucleus, then its mass is given by the number of nucleons present in it. This will be given as the mass number A.
 
Now, one atom cannot be considered in real life. So, a standard volume of one mole is taken. In one mole there are Avogadro's number of atoms. Hence, if we want to calculate the mass of a single nucleus, we will divide the mass number by the Avogadro's number.
 
begin mathsize 14px style Mass space of space straight a space nucleus equals fraction numerator Mass space number over denominator Avogadro apostrophe straight s space number end fraction equals straight A over straight N subscript straight A end style
Also, note that the mass number is not taken as 3. The mass number A from the numerator and the denominator cancel each other out, and the denominator has a value 4/3. So, the 3 comes to the numerator.
 
begin mathsize 14px style straight rho equals fraction numerator straight A over denominator straight N subscript straight A begin display style 4 over 3 end style πr cubed end fraction equals fraction numerator straight A over denominator straight N subscript straight A begin display style 4 over 3 end style straight pi open parentheses straight r subscript 0 straight A to the power of 1 divided by 3 end exponent close parentheses cubed end fraction equals fraction numerator straight A over denominator straight N subscript straight A begin display style 4 over 3 end style πr subscript 0 superscript 3 straight A end fraction
Here comma space straight A space and space straight A space get space cancelled space and space 3 space from space the space denominator space of space denominator space goes space to space the space numerator.
So comma space we space get
straight rho equals fraction numerator 3 over denominator 4 πN subscript straight A straight r subscript 0 superscript 3 end fraction end style

Answered by Romal Bhansali | 19th Jul, 2016, 04:14: PM