Molarity is a measure of the concentration of a solution, defined as the number of moles of solute per liter of solution. It is expressed in units of moles per liter (mol/L).
Calculating Molarity
To calculate the molarity of a solution, use the following formula:
Molarity = Moles of Solute / Volume of Solution (in liters)
Example:
If you have 0.5 moles of NaCl dissolved in 2 liters of water, the molarity of the solution is:
Molarity = 0.5 moles / 2 liters = 0.25 mol/L
Diluting AP CHEM solutions
When diluting a solution, the total number of moles of solute remains the same. However, the volume of the solution increases, resulting in a decrease in molarity.
Dilution Formula:
$$ M_1V_1 = M_2V_2
$$ * $ M_1 $ is the initial molarity
- $ V_1 $ is the initial volume
- $ M_2 $ is the final molarity
- $ V_2 $ is the final volume
Example:
If you have 500 mL of a 1.0 M NaCl solution and want to dilute it to a final volume of 1 liter, the final molarity will be:
$$ (1.0 ; M)(500 ; mL) = (M_2)(1000 ; mL)
$$ $$ M_2 = 0.5; M
$$
Stoichiometry and Molarity
Molarity can be used in stoichiometric calculations to determine the number of moles of reactants or products involved in a chemical reaction.
Example:
If you have 100 mL of a 0.1 M HCl solution and react it with 100 mL of a 0.1 M NaOH solution, the reaction will produce NaCl and water. The moles of HCl and NaOH used are:
Moles of HCl = (0.1 mol/L)(100 mL) = 0.01 mol
Moles of NaOH = (0.1 mol/L)(100 mL) = 0.01 mol
Additional Notes:
- Molarity is Temperature-dependent.
- Molarity can be converted to other concentration units, such as normality (N) and parts per million (ppm).
- Molarity is a useful measure of concentration for many chemical applications, including titrations, dilutions, and stoichiometric calculations.