Molarity

Molarity

A concentration given in units of molarity is calculated according to:

The unit volume is most commonly in liters. The abbreviation for molarity is M.

helpful hint:

Let's look at some examples:

1.26 g of AgNO₃ is dissolved in H₂O and the solution is diluted to 250 mL. What is the molarity?

First step: Determine the G.F.W. of AgNO₃

Second step:

Try this one at home:

Starting with this 0.0297 M solution, calculate how much volume of it you would need to prepare 100 mL of a 0.01 M solution of AgNO₃.

To do this problem you will need to (1) calculate the number of moles needed in 100 mL of solution, and (2) calculate the volume of 0.0297 M solution that contains the number of moles needed. You should obtain 33.7 mL as your final answer.

Formality

Many compounds undergo dissociation of complex formation when dissolved. For example, the solute acetic acid, CH₃CO₂H discociates slightly when dissolved in H₂O according to:

If we dissolve 0.100 moles of CH₃CO₂H in 1.000 L of H₂O we find that 1.3% of it dissociates according to:

Because of dissociation the final CH₃CO₂H concentration is less than 0.100 M. Thus, we use the term formality (F) to indicate the total number of moles of a solute in one liter of solution.

In this example the solution is a 0.0987 M solution of acetic acid, and at the same time is a 0.100 F solution of acetic acid. If there is no dissociation then molarity and formality are equivalent.