pH & Indicators Flashcards
Define pH
pH is defined as minus the log to the base 10 of the hydrogen ion concentration (mol/L)
Equation is pg 64 of log tables
What is the pH scale?
The pH scale is a scale that runs from 0 to 14 that indicates the of acidity or alkalinity of a substance.
Give three limitations of the pH scale
1) The pH scale is only valid in the 0-14 range, pH values outside this range are possible
2) The pH scale only applies to aqueous solutions – acids and bases can exist in other solutions
3) The pH scale does not work with very concentrated solutions
Finding pH of a strong acid acid
- Ensure units are in mol/L
- write dissociation equation for acid
- Use molar ratio from equation to find H+ conc.
- Sub into formulas pg 64 log tables :
pH = – log10 [H+]
Finding pH of a weak acid
- units in mols/L
- find H ion conc. by using : [H+] = √K𝑎[HA]
- Sub ans into : pH = – log10 [H+]
Finding pH of a strong base
Ensure units are in mol/L
- write dissociation equation for base
- Use molar ratio from equation to find OH- conc.
- Sub into formulas pg 64 log tables :
pOH = – log10 [OH–]
- Use pOH to find pH:
pH= 14–pOH
Finding pH of a weak base
- Ensure units are in mol/L
- OH- conc. by subbing into : √K𝚋 [BOH]
- Sub this value into : pOH = – log10 [OH–]
-FInd pH - pH= 14–pOH
Finding concentration of a strong acid
(1) Find [H+] from pH : antiLog(pH)
(2) Use [H+] to find concentration of acid
- monobasic acid: [H+] = acid molarity X 1
- dibasic acid: [H+] = acid molarity X 2
- Tribasic [H+] = acid molarity X 3
Finding Conc. of weak acid
(1) Find [H+] from pH : antiLog(pH)
(2) Use [H+] to find concentration of acid
Use [H+] = √K𝑎[HA]
Finding conc. of strong base
(1) Use pH to find pOH pH= 14–pOH
(2) Find [OH –] from pOH : [OH-]=antilog10(-POH)
(3) Use [OH-] to find concentration of base
-monobasic base: [OH –] = base molarity X 1
- dibasic acid: [OH –] = base molarity X 2
- Tribasic : [OH –] = base molarity X
Indicators
indicators are substances which change colour according to the pH of the solution in which they are placed.
How does indicators work
Most indicators are weak acids = Hln ⋚ H⁺ + In⁻
(RED) ⋚ (BLUE)
- If acid is added to the above equilibrium system then there is an increase in the number of H⁺ ions so according to le Chateliers Principle the position of equilibrium will move two the left favouring the reverse reaction so as to use up the extra H⁺, so the red colour will appear
- If a base is added to the above system then there is an increase in the OH- ions so these will react with the H⁺ ions thus decreasing the H⁺ ion concentration, so the position of equilibrium will move to the right favouring the forward reaction so as to replace the H⁺ ions removed, hence a blue colour appears
Choosing an indicator for a Strong Acid & Strong Base Titration
In this titration there will be a large range in the pH units, ranging from 3-10 units , this means ANY indicator will work.\
Phenolphthalein (range 8-10) ; colourless to pink
or
Methyl Orange (range 3-5) ; Red - yellow
Titrating a strong acid against a weak base
Methyl orange indicator, (range 3-5)
In this type of titration there will be a change in the pH units, ranging from 3-7.
-This change occurs in the acidic part of the pH scale, so we need an indicator that will change colour in acidic conditions.
Titrating a weak acid against a strong base
Phenolphthalein indicator
Change in pH units from 8-10 , occurs in alkaline part of scale , need indicator that changes in alkaline conditions
