Question

You mix 2 moles of hbr with 3 moles of koh in enough water to make 1 l of solution. What is the ph of your final solution?.

Answer 1

Taking into account the definition of pH and pOH, limiting reagent and reaction stoichiometry, the pH of the final solution is 14.

Limiting reagent and excess of moles.

Hydrobromic acid and potassium hydroxide react in a 1:1 mole ratio to produce aqueous potassium bromide and water, following the reaction:

HBr + KOH → KBr + H₂O

So, if you mix equal numbers of moles of each reactant, both the acid and the base will be completely consumed.

But in this case you mix 2 moles of HBr with 3 moles of KOH.

The limiting reagent is one that is consumed first in its entirety, determining the amount of product in the reaction. When the limiting reagent is finished, the chemical reaction will stop.

In this case, as the reaction has a 1:1 mole ratio hydrobromic acid will act as the limiting reagent, and it will be completely consumed before all the moles of potassium bromide are consumed, so there will be an excess of moles of potassium bromide.

After the reaction is complete, the resulting solution will contain an excess of moles of potassium bromide calculated as, considering that when 2 moles of hydrobromic acid are consumed, 2 moles of potassium hydroxide are consumed:

3 moles that you had at the beginning of the reaction - 2 moles that react= 1 mole of potassium bromide

Definition of molarity

Molarity is a measure of the concentration of a solute in a solution and indicates the number of moles of solute that are dissolved in a given volume.

The molarity of a solution is calculated by dividing the moles of solute by the volume of the solution:

$Molarity= \frac{number of moles}{volume}$

Molarity of potassium bromide

In this  case you know:

• number of moles= 1 mole of potassium bromide
• volume= 1 L

So, the concentration of potassium bromide is calculated as:

$Molarity= \frac{1 mole}{1 L}= 1 \frac{mole}{L}$

Definition of pH

pH is a measure of acidity or alkalinity that indicates the amount of hydrogen ions present in a solution or substance.

The pH is defined as the negative base 10 logarithm of the activity of hydrogen ions, that is, the concentration of hydrogen ions or H₃O⁺:

pH= - log [H⁺]= - log [H₃O⁺]

Definition of pOH

pOH is a measure of hydroxyl ions in a solution and is expressed as the logarithm of the concentration of OH⁻ ions, with the sign changed:

pOH= - log [OH⁻]

Relationship between pH and pOH

The following relationship can be established between pH and pOH:

pOH + pH= 14

Calculation of pH in this case

In this case, the concentration of hydroxide anions are delivered to the solution by the potassium hydroxide in a 1:1 mole ratio.

So, being the concentration of potassium bromide 1 $\frac{mole}{liters}$, the concentration [OH⁻]= 1 $\frac{mole}{liters}$.

Replacing in the definition of pOH, then the pOH is:

pOH= - log 1 $\frac{mole}{liters}$.

Solving:

pOH= 0

Finally, replacing in the relationship between pH and pOH:

pH + 0=14

pH=14

In summary, the pH of the final solution is 14.

Learn more about

molarity:

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pH and pOH:

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