you need
  • Table D.Mendeleev, pen, calculator
If you carefully examine the table of chemical elements Dmitri Mendeleev, you can see that it has the form of an apartment high-rise building, in which there are "residents" - the chemical elements.Each of them has a name (title) and the chemical symbol.And each of the elements of living in your own apartment, and therefore has a serial number.This information is presented in all the cells of the table.
But there is another figure, at first glance,
quite incomprehensible.Moreover, it contains multiple value after the decimal point that is made for greater accuracy.It is at that number and it is necessary to pay attention, because this is the relative atomic mass.And this characteristic - a constant that does not need to store and can be found in the table.By the way, even on the exam table in chemistry DIMendeleev is a reference material available for use, and each is individually package - Kim.
The molecular weight, but rather a relative molecular mass of the substance designated by the letters (Mr) the sum of the relative atomic mass (Ar), forming a molecule elements.Relative atomic mass - is just that, a mysterious figure that is in every cell of the table.To calculate these values ​​must be rounded up.The only exception is a chlorine atom, whose relative atomic mass is 35.5.The units of measure the response has not.
Example 1. Find a molecular weight potassium hydroxide (KOH)
potassium hydroxide molecule composed of one atom of potassium (K), one atom of oxygen (O) and a single hydrogen atom (H).Therefore, we find:
Mr (KOH) = Ar (K) + Ar (O) + Ar (H)
In Table D.Mendeleev find the values ​​of the relative atomic masses of elements:
Ar (K) = 39, Ar (G) = 16, Ar (H) = 1
From: Mr (KOH) = 39 + 16 + 1 = 56
Example 2. Find molecular weight sulfuric acid (H2SO4 two ash-es of four)
sulfuric acid molecule consists of two atoms of hydrogen (H), one sulfur atom (S) and four oxygen atoms (O).Hence, we find:
Mr (H2SO4) = 2Ar (H) + Ar (S) + 4Ar (O)
In Table D.Mendeleyev find the values ​​of the relative atomic masses of elements:
Ar (K) = 39, Ar (G) = 16, Ar (H) = 1
Hence: Mr (H2SO4) = 2 x 2 + 32 + 4 x 16 = 98