Mass Calculator from Molarity

Molecular weight basics

Molar mass, also called molecular weight, is defined as the mass of a substance relative to the number of molecules or atoms it contains, measured in moles. The molar mass of a substance is calculated as the weighted average of the naturally occurring isotopes and is expressed in grams per mole. In chemistry, a mole represents a specific quantity of a substance containing Avogadro's number (6.02214076 × 10²³) of components, such as atoms or molecules. In other words, one mole (6.02214076 × 10²³ molecules) of a substance has a mass equal to its molecular weight in grams. An example of calculation of the molecular weight of methane is demonstrated below.

Figure 1. Chemical structure of methane and example calculation of its molecular weight.

Mass calculation from molarity

Molarity is defined as the number of moles of solute dissolved in one liter of solution and is expressed in units of moles per liter (M). So, if one mole of a substance was dissolved in one liter of solution, it would be a 1 Molar solution. When working in the laboratory, high molarity/high concentration stock solutions are often prepared and diluted to lower molarity/lower concentration working solutions. To prepare a stock solution at a desired molarity and volume, the mass of the solute can be calculated using the equation below:

Preparing stock solutions

Stock solutions are high concentration/high molarity solutions that are prepared to save time, resources, and space in the laboratory and can be diluted to working solutions for use in multiple experiments. Often, stock solutions need to be prepared at a specific pH. Therefore, when preparing stock solutions a general protocol including pH measurement should be followed for accurate solution preparation.

First, weigh out the required amount of the compound calculated using VectorBuilder’s molarity calculator and add it to a clean flask, beaker, or bottle. Next, slowly add 70-80% of the total volume of solvent and agitate the solute with a magnetic stir bar or manual agitation. After the compound has dissolved, a pH meter can be placed in the solution followed by adjusting the solution to its proper pH. When the desired pH of the solution is achieved, the stock solution can be adjusted to its final volume.

To calculate the amount of stock solution needed to prepare a working solution, use VectorBuilder’s Dilution Calculator.