If you examined the human body on a chemical composition basis, which of the following combinations of elements would be most common?
a) o, c, n, na
b) c, h, ca, cl
c) o, c, h, n
d) c, n, ca, s
e) o, c, p, s

C) O, C, H, N

Explanation:

Roughly 96% of the mass of the human body is made up of the four elements which are carbon, oxygen, hydrogen and nitrogen.

The elements, oxygen and hydrogen are found predominantly in water, which makes up 60% of body's weight approximately. Element carbon is synonymous with life. The central role of the element, carbon is due to the fact that the carbon has 4 bonding sites which allow the element for the building of long and the complex chains of molecules. Nitrogen is found in the organic molecules, including the amino acids which make up proteins, and nucleic acids which make up DNA.

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"