Calculating the molecular weight of a solution is slightly different than calculating the molecular weight of a single compound. A solution is a homogeneous mixture of two or more substances, a solute dissolved in a solvent. Therefore, we don't calculate a single molecular weight for the entire solution. Instead, we determine the molecular weight of each component (solute and solvent) separately, and then we can calculate other properties related to the concentration of the solution (like molarity or molality). The video [How To Calculate Molecular Weight and Molar Mass! - YouTube](insert YouTube link here if available) explains how to calculate the molecular weight of individual compounds.
Calculating Molecular Weight of Individual Components
The molecular weight (also known as molar mass) of a compound is the sum of the atomic weights of all atoms in its chemical formula. You find the atomic weights of the elements on a periodic table.
Steps:
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Identify the chemical formula: Determine the precise chemical formula of the solute and the solvent. For example, for a solution of glucose (C₆H₁₂O₆) in water (H₂O).
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Find atomic weights: Use a periodic table to look up the atomic weight of each element in the formula.
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Calculate the molecular weight: Multiply each element's atomic weight by the number of times it appears in the formula and then sum the results.
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Example (Glucose):
- Carbon (C): 12.01 g/mol × 6 = 72.06 g/mol
- Hydrogen (H): 1.01 g/mol × 12 = 12.12 g/mol
- Oxygen (O): 16.00 g/mol × 6 = 96.00 g/mol
- Total Molecular Weight of Glucose: 72.06 + 12.12 + 96.00 = 180.18 g/mol
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Example (Water):
- Hydrogen (H): 1.01 g/mol × 2 = 2.02 g/mol
- Oxygen (O): 16.00 g/mol × 1 = 16.00 g/mol
- Total Molecular Weight of Water: 2.02 + 16.00 = 18.02 g/mol
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Calculating Solution Properties
Once you know the molecular weight of the solute and solvent, you can use this information to calculate various properties of the solution:
- Molarity (M): Moles of solute per liter of solution. Calculators are available online (see Sigma-Aldrich Molarity Calculator for example) to assist in this calculation.
- Molality (m): Moles of solute per kilogram of solvent.
- Weight percent (% w/w): Grams of solute per 100 grams of solution.
- Mole fraction (χ): Moles of a component divided by the total moles of all components.
These calculations require knowing the mass and/or volume of both solute and solvent in your solution.
