What is molecular weight (mass)?

Question

What would be the molecular weight/mass for a Hydrogen Peroxide molecule?

What would be the molecular weight/mass for a potassium chloride molecule?

Answer 1

All atoms are made of protons and neutrons in the nucleua; surrounded by a number of electrons equal to the protons ( except in an ion, where the number of electrons may be less or more).

All protons, neutrons and electrons are identical.  The only thing that varies from element to element is the number of protons in the nucleus (and the number of electrons).  This is what causes  the elementsd to have different characteristics.

Because protons and neutrons are very much heavier than the electrons, virtually all the mass of an atom is concentrated in the nucleus.  Therefore, knowing how many protons and neutrons there are in the nucleus will enable you to calculate the relative mass of each aom.  

Hydrogen, with one proton and no neutron, has mass 1

carbon, with 6 protons annd 6 neutrons has mass 12.

and so on.

But when you ;look at the periodic table; you will notice that the masses for most elements are not whole numbers.  This is because, unlike protons, the number of neutrons in an atom may vary (f0known as isotopes, which have identical chemical properties).

For example,  Iron has mass 55.84. Iron is element 26; so why isn't its mass 52? (265 protons and 26 neutrons).  The reason is that iron can exist in more than one stable form, with 27, 30, 31 or 32  neutrons.  In a natural form, the average  works out to be a little less than 30; hence the atomic mass is 55.84.

Atomic masses are compared to oxygen (mass 16.00), because oxygen has only one stable isotope.

molecular mass is simply determined by adding up the atomic masses of all the atoms in the molecule.  eg, water has molecular mass 18 (16 for oxygen, one each for the hydrogen).