.........Introduction to fundamental concepts of chemistry
An error is defined as:                                                                     
"The difference between the measured value and the actual value."
   If two persons use the same instrument for measurement for finding the same measurement, it is not    essential that they may get the same results. There may arises a difference between their    measurements. This difference is referred to as an "ERROR".
Types Of Error
   Errors can be divided into three categories:
   (1) Personal Error
   (2) Systematic Error
   (3) Random Error
Personal Error
   An error comes into play because of faulty procedure adopted by by the observer is called "PERSONAL    ERROR".
   Personal error comes into existence due to making an error in reading a scale. It is due to faulty procedure    adopted by the person making measurement.
Systematic Error
   The type of error arises due to defect in the measuring device is known as "SYSTEMATIC ERROR"
   Generally it is called "ZERO ERROR". it may be positive or negative error. Systematic error can be    removed by correcting measurement device.
Random Error
   The error produced due to sudden change in experimental conditions is called "RANDOM ERROR".
   For example:
   During sudden change in temperature, change in humidity, fluctuation in potential    difference(voltage).
   It is an accidental error and is beyond the control of the person making measurement.
Atomic Mass
   Atomic mass is defined as :
"The mass of one atom of the element compared with the mass of one atom of C12"
   Atomic mass is a ratio therefore it has no unit. Generally atoms mass is expressed in ATOMIC MASS    UNIT(a.m.u).
   One atomic mass unit is equal to 1/12 of the mass of a C12 atom.
Empirical Formula
"Empirical Formula is that formula which expresses the relative number
of each kind of atoms present in the molecule of a compound"

"The formula of a compound which expresses the ratio in which atoms of
different elements are combined in a molecule"

Empirical formula only indicates atomic ratios but it does not indicate actual number of atoms of different    kinds present in the molecule of a compound.
   Two or more compound may have same empirical formula.
   Empirical formula is determined by experiment.
Molecular Formula
The molecular formula of a compound is defined as:
"The formula of a compound which not only expresses the relative number of atoms of each
kind but also expresses the actual number of atoms of each element present in one molecule".
Molecular formula and empirical formula of a compound are related as:
   Where "n" is an integer and is given by:
n = molecular mass of compound / Empirical formula mass of compound
   Molecular formula of propane = C3H8.
   Molecular formula of sugar = C12H22O11.
Limiting Reactant
The limiting reactant is defined as:
" The substance which produces least amount of products when it is
completely consumed in a chemical reaction".