428 MedChem Quiz #1 Review
Salman Bin Abdulaziz University
Lecturer: Ali Altharawi
altharawi@gmail.com
Molarity: It is a unit of concentration measuring the number of moles of solute per liter
of solution. The unit of molarity, therefore, is mole/liter.
Molarity = (mole of solute / liter of solution) …………Eq.1
To calculate molarity, you should
1. Find the number of moles of solute dissolved in solution.
2. Find the volume of solution in liters.
3. Divide moles of solute by liters solution.
Example 1: Calculate the molarity of a solution prepared by dissolving 25 gm of KMnO4
into enough H2O to make 750 mL of solution.
1. First, find the number of moles of solute.
# Moles = (Weight of solute in gm / Molar mass or Molecular weight)
Molar mass of KMnO4 = (39.1 + 54.9 + 4(16)) = 158.0
Thus, # Moles = (25.0 / 158.0) = 0.158 moles.
2. Second, find the liters of solution that is needed. Keep in mind that the unit is
Liter. Therefore, (750 mL / 1000 mL) = 0.75 L
3. Finally, use Eq.1 to calculate the molarity of the solution.
There are several types of calculations that you need to be able to do with Molarity.
1. Calculating Molarity from Mass and Volume as in Example 1.
2. Calculating Molarity from Moles and Volume.
e.g. molarity of 0.75 mol NaCl in 4.2 L of H2O is 0.179 M.
3. Calculating Mass of Solute from Molarity.
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Example: How would you prepare 700 mL of 1.5 M solution of NaCl?
This question ask you to find the mass (amount in gm) of NaCl that is needed to
prepare 1.5 M NaCl.
x M = y Moles / z L
Substitute Known values: 1.5 M = y Moles / 0.7 L
y Moles = 1.5 x 0.7 = 1.05 Moles of NaCl.
Molar mass (Molecular weight) of NaCl = 58.5 gm/mole
Thus, 1.05 mole x 58.5 gm/mole = 61.43 gm of NaCl
Therefore, 61.43 gm of NaCl need to be dissolved in enough H2O to make 700 mL of 1.5
M NaCl solution.
4. Calculating Moles of Solute from Molarity.
Example: How many moles of NaCl are contained in 700 mL of a 1.5 M NaCl
solution?
1.5 M means that there is 1.5 Moles of NaCl in 1 Liter of H2O.
Therefore, X moles of NaCl = (1.5 moles x 0.7 L) / 1 L = 1.05 moles of NaCl.
Exercise:
1. How would you prepare 100.0 mL of 0.25 M KNO3?
2. What is the molarity of a solution if a chemist dissolved 98.4 gm of FeSO4 in
enough H2O to make 2.0 L solution?
3. How many moles of KBr are in 25.0 mL of a 1.23 M KBr solution?
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