Chemical Calculator provides a set of tools that aid in preparation of solutions from chemical substances, liquid chemicals, or concentrated stock solutions.
The calculations of molarity (molar concentration), mass, and dilution volume of a chemical substance are based on the equation:
molar concentration [mol·l^{1}]  =  mass [g]
molar mass [g·mol^{1}] · volume [l]

Note that the molar mass is always required for the calculations. The program will report an error if the molar mass value is missing.
The "Percentage and molarity" calculator provides two options: % w/v (weight in volume) and % v/v (percents of volume). The first option is used for substances dissolved in a solvent. The value describes the amount by weight (mass) of a solid substance dissolved in a measured quantity of liquid. Percent w/v expresses the number of grams of a substance in 100 ml of solution. As in this case the percentage directly defines the weight of the substance in a volume unit, the molarity can be calculated without knowing the mass density of the solution:
molar concentration [mol·l^{1}]  =  percent w/v[%] · 10
molar mass [g·mol^{1}]

In the second case, the % v/v describes the volume fraction of a liquid mixed with another liquid. To calculate the molarity of the liquids, the weights have to be derived from mass density of the solution:
molar concentration [mol·l^{1}]  =  mass density[kg·l^{1}] · percent v/v[%] · 10
molar mass [g·mol^{1}]

Again, the molar mass is always required for the calculations and the program will report an error if the molar mass value is missing.
This calculator gets the results with a simple propotion equation:
stock solution volume  =  target concentration
stock solution concentration

· target volume 
Conversions between molar concentration units and mass per volume units are done with the first equation stated on this page. Start with setting the concentration units to avoid warnings related to missing MW in cases where it is actually not needed (no unit conversion required).
Dilution of a whole solution to a certain target concentration is calculated with the formula:
volume to add  =  (  initial concentration
final concentration

 1  )  · initial volume 
Same as above, conversions between molar concentration units and mass per volume units are done with the first equation stated on this page. Start with setting the concentration units to avoid warnings related to missing MW in cases where it is actually not needed (no unit conversion required).
Absorbance of a solution (also called "optical density") depends on the concentration of a lightabsorbing solute as described by the BeerLambert law:
A = ε · l · c
A    absorbance [unitless] 
ε    molar extinction coefficient [l·mol^{1}·cm^{1}] 
l    optical path length [cm] 
c    molar concentration of the solute [mol·l^{1}] 
Note that the linear relationship described by the BeerLambert low is valid only in a certain range of concentrations (aprox. 0.2  0.8 when l = 1). Outside this range, the dependency becomes nonlinear and the calculations inaccurate.