Question

answer for this...... plz

Answer 1

By international agreement the mass of an atom of carbon-12 is given as 12 unified atomic mass units (u). Therefore, 1 u is 1/12th the mass of an atom of carbon-12.

 

In most chemistry work, you can:

• neglect the tiny contribution to atomic mass from electrons

• take both the mass of a proton and a neutron as being equal to 1 u.

 

In a pure isotope all the atoms have the same atomic structure and the same mass.

 

Relative isotopic mass is then simply the mass number of the isotope, e.g. the relative isotopic mass of 32S is 32.

 

However, most elements contain a mixture of isotopes, each with a different mass number. To find the relative atomic mass of an element we must know:

 

The relative isotopic mass of each isotope.

The proportion of each isotope in the element.

 

For example:

Copper has 2 isotopes, 63Cu and 65Cu. The proportions of each isotope are 69.17% and 30.83%.

These can be rounded to 70% and 30%.

The relative atomic mass of Copper is therefore (70/100 x 63) + (30/100 x 65) = 63.6

There is no unit as it is a relative value.

 

 

Molecular Mass (Mr) is the sum of all therelative atomic masses for all the atoms in a given formula.

 

For covalent compounds it is called the Relative Molecular Mass.

For ionic compounds it is called theRelative Formula Mass.

Answer 2

The relative molecular mass or molecular weight is the total mass of a compound. It is equal to the sum of the individual atomic masses of each atom in the molecule. It is easy to find the molecular mass of a compound with these steps.

• Determine the molecular formula of the molecule.
• Use the periodic table to determine the atomic mass of each element in the molecule.
• Multiply each element's atomic mass by the number of atoms of that element in the molecule. This number is represented by the subscript next to the element symbol in the molecular formula.
• Add these values together for each different atom in the molecule.

A relative atomic mass (also called atomic weight; symbol: Ar) is a measure of how heavy atoms are. It is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.