Limiting reagent (Limiting reactant): The reagent which limits a reaction or the reagent which is completely consumed in a chemical reaction is called limiting reagent or limiting reactant.
For e.g. in the reaction
2SO2(g) + O2(g)    2 SO3(g), 2 moles of SO2 reacts completely with 1 mole of O2 to form 2 moles of SO3. If we take 10 moles each of SO2 and O2, we get only 10 moles of SO3 because 10 moles of SO2 requires only 5 moles of O2 for the complete reaction. So here SO2 is the limiting reagent and 5 moles of O2 remains unreacted.

Reactions in solutions:
Solutions are homogeneous mixture containing 2 or more components. The component which is present in larger quantity is called Solvent and the other components are called Solutes. Or, the substance which is dissolved is called solute and the substance in which solute is dissolved is called solvent. For e.g. in NaCl solution, NaCl is the solute and water is the solvent. A solution containing only 2 components are called Binary Solution. If the solvent is water, it is called aqueous solution.

 Concentration Can be Expressed in the Following Ways:
  Ø  Mass percent (w/w or m/m): It is defined as the number of parts solute present in 100 parts by mass of solution. i.e. 
Mass % of a component =   Mass of solute  × 100
                                             Mass of solution

  Ø  Molarity (M): It is defined as the number of moles of solute dissolved per litre of solution. i.e. Molarity (M) = Number of moles of solute (n)
                               Volume of solution in litre (V)

1 M NaOH solution means 1 mole of NaOH is present in 1 L of solution.

ü      Molarity equation:    M1V1   =    M2V2
                                  (Before dilution) = (After Dilution)
Molarity of a solution decreases on increasing temperature. Molarity of pure water is 55.56 mol L-1.

  Ø  Molality (m) = It is number of moles of solute dissolved per 1000g (1kg) of solvent.
 Molality =  Number of moles of solute
                                             Mass of solvent in kg
Among the above concentration terms, Molarity depends on temperature because it is related to volume, which changes with temperature. Where as Molality is Independent of Temperature.

  Ø  Mole Fraction (x): It is defined as the ratio of the number of moles of a particular component to the total number of moles of solution. i.e.
Mole fraction of a component =  Number of moles of the component
                                            Total number of moles of all the components
Let us consider we have two components 1 and 2. Therefor mole fraction of component 1 is x1, and component 2 is x2.
x1 + x2 = 1 i.e the sum of the mole fractions of all the components in a solution is always equal to 1.