Calculating Millimolar Concentrations A Guide from Sciencing

Concentration of a dissolved compound in the solution can be calculated using several approaches. Molarity refers to a number of moles of the compound in 1 liter of the solution and expressed in molars (abbreviated as “M”). Molarity=number of moles/Volume of solution (in liters). The prefix “milli-” indicates “1 thousandth,” that is, the magnitude of 0.001 (see Resources). Hence, 1 mole is equal to 1 millimoles multiplied by 1,000. As an example, calculate concentration in millimolars if 0.5 grams of sodium hydroxide (NaOH) were dissolved in 500 ml of water.

Step 1

Calculate the molecular mass of the compound. Molecular mass is calculated as the sum of mass of all atoms in the molecule. The atomic weights of corresponding elements are given in the periodic table of the chemical elements (see Resources). In this example, M(NaOH)=M(Na)+M(O)+M(H)=23+16+1=40 g/mole.

Step 2

Calculate the amount of the component in moles using the formula, Amount (in moles) =mass (compound)/molecular mass (compound). In our example, Amount (NaOH)=0.5 g /40 g/mole= 0.0125 moles.

Step 3

Convert moles to millimoles using the following proportion: 1 mole corresponds to 1 millimole x 1,000. Amount (in moles) corresponds to Amount (in millimoles). The solution of this proportion gives rise to the formula: Amount (in millimoles)= Amount (moles) x 1,000. In our example, Amount (NaOH)= 1,000 x 0.0125 moles=12.5 millimoles.

Step 4

Calculate concentration in millimolars using the formula: Molarity (millimolars)= Amount (in millimoles)/Volume of solution (in liters). In our example, the volume of the solution is 500 ml or 0.5 liters. The molarity of NaOH would be: Molarity (NaOH)= 12.5 millimoles/0.5 liters=25 millimolars.

Things Needed

• Calculator
• Periodic table of the chemical elements