Introduction to Moles and Molarity

(Review before Lecture 2)

1. Moles 

A mole (mol) of a substance is simply its atomic or molecular weight expressed in grams (g).

A millimole (mmol) of a substance is the number of grams in a mole of the substance, divided by 1,000.  (This would be expressed in milligrams; mg)

Example 1: Atomic weight of Na = 23.  Therefore, 1.0 mol of Na = 23.0 g, 2.0 mol of Na = 46.0 g, 0.5 mol of Na = 11.5 g.

Example 2: Molecular weight of glucose (C₆H₁₂O₆) = 180.  Therefore, 1 mol of glucose = 180 g.    1 mmol of glucose would be 180 mg.

2. Molarity

Molarity (M) is the # moles of a substance per Liter (L) of solvent (mol/L).

A millimolar (mM) solution contains: #moles/1000 in 1 L of solvent (mmol/L)

Example 1: A 1 M solution of NaCl would contain 1 mol of NaCl (58 g) in 1 L of solvent (typically H₂O)

Example 2. A 2 mM solution of glucose would contain 2 mmol of glucose in 1 L of solvent (or 1 mmol glucose in 0.5 L of solvent – WHY?).

Practice Problems (use a periodic table to answer the following questions):

1.  How many grams of MgCl₂ are there in 1 Millimole (mmol) of MgCl₂?

2. You want to make a 1.5 mmol solution of NaCl.   How would you do it?