What unit is commonly used to express concentration in chemistry?

Concentration in chemistry is a measure of the amount of a substance in a given volume of solution. Molarity (M) is a widely accepted unit that expresses concentration as moles of solute per liter of solution, making it a fundamental concept for understanding reaction dynamics and stoichiometry in many chemical contexts.

Grams per liter (g/L) is another commonly used unit for concentration, particularly when dealing with mass-based measurements. This unit indicates the mass of solute present in one liter of solution and is often utilized in various practical applications in laboratories and industries.

Given that both Molarity (M) and Grams per liter (g/L) serve as effective measures for expressing concentration, the answer correctly reflects that multiple units can be appropriately used in different scenarios within chemistry. Parts per million (ppm) is specific for very dilute solutions and is often used for trace substances, thus not as general as the other two options in terms of common usage across various concentrations. Therefore, recognizing that both Molarity and grams per liter are integral in conveying the concept of concentration in chemistry justifies the choice.