Particles in Solution

If a substance is completely dissolved in a solvent, all atoms/molecules are suspended in the solution as separate particles. Knowing the number of moles of a substance dissolved in solution, the concentration of the solution can be expressed as particle count per unit-volume (instead of weight). Two of the chemical expressions of particle-count concentration are molarity and molality.

Molarity

The number of moles of solute in one liter of solution is referred to as molarity (M). For example, if one liter of solution contains 2 moles of glucose, it is a 2-molar solution. Another example: if three liters of solution contains 9 moles of glucose, it is a 3-molar solution.


Molality

The number of moles of solute per kilogram (kg) of pure solvent is referred to as molality (m). So, if two moles of glucose are dissolved in 1 kg of pure water, it is a 2-molal solution. Eight moles of glucose dissolved in 2 kg of pure water gives you a 4-molal solution.



Molarity vs. Molality


 

Exercises

1. 18 moles of sucrose are present in 3 liters of solution. What is the molarity of the solution?
2. How many grams are needed of KCl to make 750 ml of a 0.5M solution? (The molecular weight of KCl is » 74.)
3. A solution contains 116g of NaCl in 500 ml. What is the molarity? (Remember, the molecular weight of NaCl is » 58.)
4. What is the molarity of a 500 mL bag of 5% Dextrose solution? (The molecular weight of dextrose is » 180.)
5. What is the molarity of a 250ml bag of 0.9% NaCl solution? (The molecular weight of NaCl is » 58.)
6. How many mmoles are there in 250 ml bag of 1M solution of glucose? (The molecular weight of glucose is » 180.)