Overview
Molecular weight (MW) is the sum of the atomic weights of the elements that make up a molecule. In the lab, it is a necessary value for converting mass (grams) into molarity (moles per liter).
Because biological assays often require specific molar concentrations to activate or inhibit a signaling pathway, accurate use of molecular weight is fundamental to the accuracy of your results.
Key Concepts
The Mole
One mole of a substance contains 6.022 x 10^23 particles. The molar mass of a compound is defined as its mass in grams equal to its molecular weight.
Molarity (M) Calculation
To prepare a molar solution, you need to know the formula: M = moles of solute / liters of solvent. You use the MW to determine how many grams are required to reach the desired mole amount.
Formula Weight vs. Molecular Weight
Sometimes you may see Formula Weight (FW) used interchangeably with MW. FW is typically used for salts or mixtures, but the math for calculations remains the same.
Accuracy in Weighing
When working with small quantities, use an analytical microbalance. Ensure your weighing environment is free from drafts to avoid errors in mass measurement.
Practical Application
Always double-check the molecular weight on the COA provided with your compound, as hydrates or salt forms may have different weights than the pure base. Use this verified value for all your dilution calculations.
Disclaimer: All research compounds are for laboratory research use only. They are not intended for human or veterinary use.
Sources
- General Chemistry Principles, Atkins et al., 2018
- Laboratory Calculations Guide, Science Direct, 2020
- Standardized Measuring Protocols, NIST, 2021