Solving Percent Ionization

[905988526]

When solving for percent ionization, how do you know whether to use the M in the equation as -x or +x? For example if there was a problem with 0.0789M, how would you know to make the denominator 0.0789-x or 0.0789+x?

[Marta Olson 3F]

Hi!

The denominator is determined by the concentration of the reactants, and the numerator is determined by the concentration of the products. If the problem gives you 0.0789M as the concentration of the reactants, it is safe to assume that the reaction will proceed in the direction of reactants -> products until equilibrium is reached. This means that the concentration of the reactants will have to go down as it is converted to the products, so we will use subtraction. You will use 0.0789-x.

Hope this helps!

[Naya Bender 3B]

Sorry this isn't an answer, but why would you +/- the given M? I thought you always just used the given and don't change it.

[Kaiden]

Hi,

Calculating percent ionization should always be done as (concentration ions at equilibrium) / (concentration of reactant before ionization initially). This means that if the reaction started with 0.789M then that is the denominator

[Ashley_Kim_3H]

Hi,

To resolve any confusion, I believe Marta's response is directed toward a question on why we would be having (0.0789 - x) on the "E" row of the ICE table for the reactants at equilibrium. For the initial question and adding to Kaiden's response, the denominator to calculate percent ionization would be the initial concentration of reactant before ionization so it should simply be 0.0789M without any further calculations.

Hope this helps!