How to Calculate Initial Concentrations

How to Calculate Initial Concentrations
••• ludinko/iStock/GettyImages

Calculating the initial concentration of a solution – otherwise known as molarity – is an important process commonly found in the chemical and biochemical world. Molarity is the number of moles of solute per liter of solution. Therefore, you need to determine how many moles of a solute are in the solution and the total volume of the solution.

Step 1. Weigh the amount of solute (the compound being dissolved) in grams. Then determine how many grams are in a mole of the solute. There are 40 g per mole in sodium hydroxide (NaOH). Therefore, 20 g of NaOH would equal 0.50 mol of NaOH. The equation looks like this:

mol NaOH = 20.0g NaOH x 1 mol NaOH/40.0 g NaOH.

Step 2. Measure the amount of the solvent that you have. If it is less than a liter, convert the number of milliters into liters. There are 1000mL in 1L. For example, if you have 500 mL:

500 mL x 1L/1000mL = 0.500 L solvent.

Step 3. Divide the moles of solute found in Step 1 by the liters of solvent found in Step 2 to find the initial concentration of a solution. The equation looks like this:

M = 0.50 mol NaOH/0.500 L solvent = 1 M NaOH.

In this example, the molarity (M) of the NaOH in the solvent is one mole. As more of the solvent is removed, the concentration of the NaOH would continue to rise. With acids and bases, the higher the concentration, the stronger it becomes.

TL;DR (Too Long; Didn't Read)

Keep track of your units so that you can make a clean conversion to moles and the liters of the solvent. Not keeping track of units can make it difficult when dealing with conversions from very small amounts to moles.

Related Articles

How to Calculate Solubilities
How to Convert ML to MG
How to Determine Solubility Percentage
How to Convert PPM to MCG
How to Find the Mole Fraction
How to Calculate the Percent Weight Per Volume
How to Calculate the pH of Ammonia Water Using KB
How to Determine the Concentration of a Titration
How to Find Mass Percentage
How to Calculate the Osmolarity
How to Convert Milligrams Per Liter to Molarity
Why Does Sugar Affect the Freezing Point of Water?
How to Calculate Mole Fractions Using Mass Percent
How to Calculate Concentration From Density
How to Convert From Molarity to Molality
How to Convert Micromolar to PPM
How to Calculate Conductivity Due to Concentration
How to Calculate Particle Concentration
How to Calculate the Number of Moles in a Solution
How to Calculate Moles from Molecular Weight